Micro-LED light extraction efficiency enhancement

ABSTRACT

A light source includes an array of micro-light emitting diodes (micro-LEDs), an array of micro-lenses, and a bonding layer bonding the array of micro-lenses to the array of micro-LEDs. Each micro-LED of the array of micro-LEDs includes a first mesa structure formed in a plurality of semiconductor layers. The array of micro-lenses is bonded to a first semiconductor layer of the plurality of semiconductor layers by the bonding layer. The first semiconductor layer includes an array of second mesa structures formed therein. The first mesa structure and the second mesa structure are on opposite sides of the plurality of semiconductor layers. Each second mesa structure of the array of second mesa structures is aligned with a respective micro-lens of the array of micro-lenses and the first mesa structure of a respective micro-LED of the array of micro-LEDs.

BACKGROUND

Light emitting diodes (LEDs) convert electrical energy into opticalenergy, and offer many benefits over other light sources, such asreduced size, improved durability, and increased efficiency. LEDs can beused as light sources in many display systems, such as televisions,computer monitors, laptop computers, tablets, smartphones, projectionsystems, and wearable electronic devices. Micro-LEDs (“μLEDs”) based onIII-V semiconductors, such as alloys of AlN, GaN, InN, AlGaInP, otherternary and quaternary nitride, phosphide, and arsenide compositions,have begun to be developed for various display applications due to theirsmall size (e.g., with a linear dimension less than 100 μm, less than 50μm, less than 10 μm, or less than 5 μm), high packing density (and hencehigher resolution), and high brightness. For example, micro-LEDs thatemit light of different colors (e.g., red, green, and blue) can be usedto form the sub-pixels of a display system, such as a television or anear-eye display system.

SUMMARY

This disclosure relates generally to micro-light emitting diodes(micro-LEDs). More specifically, and without limitation, disclosedherein are techniques for improving the collected light extractionefficiencies (LEEs) of micro-LEDs, such as AlGaInP-based redlight-emitting micro-LEDs. Various inventive embodiments are describedherein, including devices, systems, methods, structures, materials,processes, and the like.

According to certain embodiments, a light source may include an array ofmicro-light emitting diodes (micro-LEDs), where each micro-LED of thearray of micro-LEDs may include a first mesa structure formed in aplurality of semiconductor layers. The light source may also include anarray of micro-lenses, and a bonding layer bonding the array ofmicro-lenses to a first semiconductor layer of the plurality ofsemiconductor layers. The first semiconductor layer may include an arrayof second mesa structures formed therein, where each second mesastructure of the array of second mesa structures may be aligned with arespective micro-lens of the array of micro-lenses and the first mesastructure of a respective micro-LED of the array of micro-LEDs.

In some embodiments of the light source, a refractive index of the arrayof micro-lenses may be equal to or greater than a refractive index ofthe first semiconductor layer. An optical thickness of the bonding layerbetween each second mesa structure and a corresponding micro-lens of thearray of micro-lenses may be less than about ⅕ (e.g., about 1/10 or1/20) of a center wavelength of light emitted by the array ofmicro-LEDs. A maximum optical thickness of the bonding layer may beequal to an integer multiple of a half-wavelength of a center wavelengthof light emitted by the array of micro-LEDs. In some embodiments, awidth of the second mesa structure may be smaller than a width of thefirst mesa structure, and the second mesa structure may include verticalor tilted sidewalls. The refractive index of the bonding layer may belower than a refractive index of the first semiconductor layer. In someembodiments, the bonding layer may include SiO₂, SiN, or a transparentconductive oxide (TCO).

In some embodiments of the light source, the array of micro-lenses andthe first semiconductor layer may include AlGaInP. In some embodiments,a pitch of the array of micro-LEDs may be equal to or less than about 2μm, a width of the first mesa structure may be equal to or less thanabout 1.2 μm, and a width of the second mesa structure may be equal toor less than about 0.8 μm. A ratio between a width of a micro-lens ofthe array of micro-lenses and a width of the second mesa structure maybe greater than about 2. In some embodiments, each second mesa structureof the array of second mesa structures may be at a focal point of therespective micro-lens of the array of micro-lenses. In some embodiments,the plurality of semiconductor layers may include a p-dopedsemiconductor layer, an active layer configured to emit visible light,and an n-doped semiconductor layer, where the first semiconductor layermay include the p-doped semiconductor layer or the n-doped semiconductorlayer. In some embodiments, each micro-LED of the array of micro-LEDsmay include a passivation layer on sidewalls of the first mesastructure, and a back reflector coupled to a second semiconductor layerof the plurality of semiconductor layers and electrically connected to abackplane wafer.

According to certain embodiments, a micro-LED device may include abackplane wafer including electrical circuits fabricated thereon, anarray of micro-LEDs bonded to the backplane wafer, an array ofmicro-lenses, and a bonding layer bonding the array of micro-lenses tothe array of micro-LEDs. Each micro-LED of the array of micro-LEDs mayinclude a first mesa structure formed in a first side of a plurality ofsemiconductor layers facing the backplane wafer, and a second mesastructure formed in a second side of the plurality of semiconductorlayers, where a center of the second mesa structure may be aligned witha center of the first mesa structure. The bonding layer may bond thearray of micro-lenses to the second mesa structures of the array ofmicro-LEDs.

In some embodiments of the micro-LED device, a refractive index of thearray of micro-lenses may be equal to or greater than a refractive indexof the plurality of semiconductor layers. An optical thickness of thebonding layer between the second mesa structure and a correspondingmicro-lens of the array of micro-lenses may be less than about ⅕ of acenter wavelength of light emitted by the array of micro-LEDs. A maximumoptical thickness of the bonding layer may be equal to an integermultiple of a half-wavelength of a center wavelength of light emitted bythe array of micro-LEDs. A refractive index of the bonding layer may belower than a refractive index of the plurality of semiconductor layers.In some embodiments, a width of the second mesa structure may be smallerthan a width of the first mesa structure, and a ratio between a width ofa micro-lens of the array of micro-lenses and the width of the secondmesa structure may be greater than about 2. The second mesa structuremay be at a focal point of a corresponding micro-lens of the array ofmicro-lenses.

This summary is neither intended to identify key or essential featuresof the claimed subject matter, nor is it intended to be used inisolation to determine the scope of the claimed subject matter. Thesubject matter should be understood by reference to appropriate portionsof the entire specification of this disclosure, any or all drawings, andeach claim. The foregoing, together with other features and examples,will be described in more detail below in the following specification,claims, and accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Illustrative embodiments are described in detail below with reference tothe following figures.

FIG. 1 is a simplified block diagram of an example of an artificialreality system environment including a near-eye display according tocertain embodiments.

FIG. 2 is a perspective view of an example of a near-eye display in theform of a head-mounted display (HMD) device for implementing some of theexamples disclosed herein.

FIG. 3 is a perspective view of an example of a near-eye display in theform of a pair of glasses for implementing some of the examplesdisclosed herein.

FIG. 4 illustrates an example of an optical see-through augmentedreality system including a waveguide display according to certainembodiments.

FIG. 5A illustrates an example of a near-eye display device including awaveguide display according to certain embodiments.

FIG. 5B illustrates an example of a near-eye display device including awaveguide display according to certain embodiments.

FIG. 6 illustrates an example of an image source assembly in anaugmented reality system according to certain embodiments.

FIG. 7A illustrates an example of a light emitting diode (LED) having avertical mesa structure according to certain embodiments.

FIG. 7B is a cross-sectional view of an example of an LED having aparabolic mesa structure according to certain embodiments.

FIGS. 8A-8D illustrate an example of a method of hybrid bonding forarrays of LEDs according to certain embodiments.

FIG. 9 illustrates an example of an LED array with secondary opticalcomponents fabricated thereon according to certain embodiments.

FIG. 10A illustrates an example of a method of die-to-wafer bonding forarrays of LEDs according to certain embodiments.

FIG. 10B illustrates an example of a method of wafer-to-wafer bondingfor arrays of LEDs according to certain embodiments.

FIGS. 11A-11F illustrate an example of a method of fabricating amicro-LED device using alignment-free metal-to-metal bonding andpost-bonding mesa formation.

FIGS. 12A-12E illustrate an example of a process of fabricating amicro-LED device according to certain embodiments.

FIG. 13 illustrates light refraction at an interface between a micro-LEDand another medium that may have a lower refractive index than thesemiconductor material of the micro-LED.

FIG. 14A illustrates an example of a micro-LED device including amicro-LED and a micro-lens fabricated in a low-refractive index materiallayer.

FIG. 14B includes a graph illustrating light transmittance as a functionof the angle of incidence at an interface between an AlInGaP-basedmicro-LED and a SiN micro-lens as shown in FIG. 14A.

FIG. 14C includes a graph illustrating light transmittance as a functionof the angle of emission at an interface between an AlInGaP-basedmicro-LED and a SiN micro-lens as shown in FIG. 14A.

FIG. 15A illustrates an example of a micro-LED device including anAlInGaP-based micro-LED and a micro-lens fabricated in an AlInGaP layerand bonded to the AlInGaP-based micro-LED through a dielectric bondinglayer.

FIG. 15B illustrates another example of a micro-LED device including anAlInGaP-based micro-LED and a micro-lens fabricated in an AlInGaP layerand bonded to the AlInGaP-based micro-LED through a bonding layer.

FIGS. 16A-16C illustrate light transmittance from an AlInGaP-basedmicro-LED to an AlInGaP micro-lens, through SiN bonding layers ofdifferent thicknesses, as a function of the angle of incidence in amicro-LED device.

FIGS. 17A and 17B illustrate light transmittance from an AlInGaP-basedmicro-LED to an AlInGaP micro-lens, through SiO₂ bonding layers ofdifferent thicknesses, as a function of the angle of incidence in amicro-LED device.

FIG. 18A illustrates an example of a micro-LED device including anAlInGaP-based micro-LED and a micro-lens fabricated in an AlInGaP layerand bonded to the AlInGaP-based micro-LED through a SiO₂ bonding layer.

FIG. 18B illustrates a beam profile of the light beam emitted by anexample of the micro-LED device of FIG. 18A.

FIG. 19A illustrates an example of a micro-LED device including anAlInGaP-based micro-LED and a micro-lens fabricated in an AlInGaP layerand bonded to the AlInGaP-based micro-LED through a bonding layer thathas a low thickness at the center of the micro-LED device according tocertain embodiments.

FIG. 19B illustrates an example of a micro-LED device including anAlInGaP-based micro-LED and a micro-lens fabricated in an AlInGaP layerand bonded to the AlInGaP-based micro-LED through a bonding layer thathas a low thickness at the center of the micro-LED device according tocertain embodiments.

FIG. 19C illustrates a beam profile of the light beam emitted by anexample of the micro-LED device of FIG. 19A, where the bonding layer mayinclude SiO₂.

FIG. 19D illustrates a beam profile of the light beam emitted by anexample of the micro-LED device of FIG. 19A, where the bonding layer mayinclude SiN.

FIG. 20 is a simplified block diagram of an electronic system of anexample of a near-eye display according to certain embodiments.

In the appended figures, similar components and/or features may have thesame reference label. Further, various components of the same type maybe distinguished by following the reference label by a dash and a secondlabel that distinguishes among the similar components. If only the firstreference label is used in the specification, the description isapplicable to any one of the similar components having the same firstreference label irrespective of the second reference label.

DETAILED DESCRIPTION

This disclosure relates generally to micro-light emitting diodes(micro-LEDs). More specifically, and without limitation, disclosedherein are techniques for improving the collected light extractionefficiencies (LEEs) of micro-LEDs, such as AlGaInP-based redlight-emitting micro-LEDs. Various inventive embodiments are describedherein, including devices, systems, methods, structures, materials,processes, and the like.

Augmented reality (AR) and virtual reality (VR) applications may usenear-eye displays that include tiny monochrome light emitters, such asmini- or micro-LEDs. In light emitting diodes (LEDs), photons may begenerated through the recombination of electrons and holes within anactive region (e.g., including one or more semiconductor layers that mayform one or more quantum wells). The proportion of the carriers (e.g.,electrons or holes) injected into the active region of an LED among thecarriers that pass through the LED is referred to as the carrierinjection efficiency. The ratio between the number of emitted photonsand the number of carriers injected into the active region is referredto as the internal quantum efficiency (IQE) of the LED. Light emitted inthe active region may be extracted from the LED at a certain lightextraction efficiency (LEE). The ratio between the number of emittedphotons extracted from the LED and the number of electrons passingthrough the LED is referred to as the external quantum efficiency (EQE)of the LED, which describes how efficiently the LED converts injectedcarriers into photons that are extracted from the LED. The EQE may be aproduct of the carrier injection efficiency, the IQE, and the LEE. InLEDs for ear-eye display, only light that is emitted from an LED into acertain direction and/or within a certain emission angle range (e.g.,within about)±18.5° may be collected by the display optics of thenear-eye displays. The proportion of the emitted photons that areextracted from the LED and are collected by the display optics may bereferred to herein as the collected LEE. For LEDs with reduced physicaldimensions, such as micro-LEDs, the IQEs, collected LEEs, and EQEs maybe very low. Improving the efficiencies of the micro-LEDs can bechallenging.

The internal quantum efficiency of an LED depends on the relative ratesof competitive radiative (light producing) recombination andnon-radiative (lossy) recombination that occur in the active region ofthe LED. Non-radiative recombination processes in the active regioninclude Shockley-Read-Hall (SRH) recombination at defect sites, andelectron-electron-hole (eeh) and/or electron-hole-hole (ehh) Augerrecombination. The Auger recombination is a non-radiative processinvolving three carriers, which affects all sizes of LEDs. Inmicro-LEDs, because the lateral size (e.g., diameter or width) of eachmicro-LED may be comparable to the minority carrier diffusion length, alarger proportion of the total active region may be within the minoritycarrier diffusion length from the LED sidewall surfaces where the defectdensity and the defect-induced non-radiative recombination rate may behigh. Therefore, a larger proportion of the injected carriers maydiffuse to the regions near the sidewall surfaces, where the carriersmay be subjected to a higher SRH recombination rate. This may cause theefficiency of the LED to decrease (in particular, at low currentinjection), cause the peak efficiency of the LED to decrease, and/orcause the peak efficiency operating current to increase. Increasing theinjected current may cause the efficiencies of the micro-LEDs to dropdue to the higher eeh or ehh Auger recombination rate at a highercurrent density, and may also cause spectral shift of the emitted light.As the physical sizes of LEDs are further reduced, efficiency losses dueto surface recombination near the etched sidewall facets that includesurface imperfections may become much more significant. III-phosphidematerials, such as AlGaInP, can have a high surface recombinationvelocity and minority carrier diffusion length. For example, carriers inAlGaInP can have high diffusivity (mobility), and AlGaInP may have anorder of magnitude higher surface recombination velocity thanIII-nitride materials. Thus, the internal and external quantumefficiencies of AlGaInP-based red light-emitting LEDs may drop even moresignificantly as the device size reduces.

In addition, at the light-emitting surface of an LED, such as theinterface between the LED and air, incident light with incident anglesgreater than a critical angle may be reflected back to the LED due tototal internal reflection (TIR). Because of the geometry of the LED,some light reflected back to the LED may be trapped and eventually beabsorbed by the LED. For example, some trapped light may be absorbed bythe semiconductor materials to generate electron-hole pairs, which mayrecombine radiatively or non-radiatively. Some trapped light may beabsorbed by metals (e.g., metal contacts or reflectors) at the bottomand/or sidewalls of the LED due to, for example, surface plasmonresonance that may be excited by p-polarized light at the interfacebetween a metal layer and a dielectric layer (e.g., the passivationlayer). In III-phosphide-based LEDs, such as some red light-emittingIII-phosphide LEDs, the refractive indices of the III-phosphidesemiconductor materials (e.g., GaP, InP, GaInP, or AlGaInP) may begreater than about 3.0 (e.g., about 3.4 or 3.5) for visible light, muchhigher than the refractive indices of many III-nitride semiconductormaterials (e.g., about 2.4 for GaN). Therefore, the critical angle fortotal internal reflection at the interface between the III-phosphidesemiconductor material and an adjacent lower refractive index material(e.g., air or a dielectric) may be much smaller than the critical anglefor total internal reflection at the interface between a III-nitridesemiconductor material and the lower refractive index material. As such,more light emitted in the active region of a III-phosphide-based LED maybe trapped in the LED due to TIR and may eventually be absorbed by theLED. Therefore, the LEE of a red light-emitting III-phosphide LED may below.

Micro-lenses may be used to collimate light emitted from LEDs toincrease the total LEEs (e.g., for extracted light with emission angleswithin ±90°) and the collected LEEs (e.g., for extracted light withemission angles within ±18.5°) of LEDs in a near-eye display. Non-nativelenses made from, for example, SiN, SiO₂, or organic materials, may beeasier to fabricate than native lenses fabricated in a thicksemiconductor layer of an LED, but may exhibit lower LEEs compared withnative lenses due to, for example, the refractive index mismatch betweenthe non-native lens and the LED, which may cause Fresnel reflection (andtotal internal reflection) at the interface between the LED and thenon-native lens.

According to certain embodiments, to improve the LEE, in particular, thecollected LEE, a micro-LED device may include a micro-lens bonded to asemiconductor layer of a micro-LED through a thin bonding layer (e.g.,SiN, SiO₂, or a transparent conductive oxide, such as indium tin oxide).The micro-lens may have a refractive index close to or greater than therefractive index of the semiconductor layer, and thus may preserve orreduce the emission angles of the light emitted by the micro-LED. Amaximum optical thickness of the bonding layer may be, for example, aninteger multiple of a half-wavelength (λ/2) of the light emitted by themicro-LED, to filter incident light with large incident angles, therebycontrolling the emission angles of the emitted light. In someembodiments, the semiconductor layer may have an uneven top surface,where the portion of the semiconductor layer at the center region of themicro-LED may have a mesa structure that has a higher thickness.Therefore, the bonding layer between the micro-LED and the micro-lensmay have a lower thickness (e.g., less than about 30 nm) that maypromote frustrated total internal reflection at the center region of themicro-LED device, such that a larger portion of the light emitted in themicro-LED may enter the micro-lens through the bonding layer (which mayhave a lower refractive index). The mesa structure of the semiconductorlayer at the center region of the micro-LED may have a size smaller thanthe active region of the micro-LED. Light emitted in the active regionof the micro-LED may be concentrated in the smaller-sized mesa structureof the semiconductor layer at the center region of the micro-LED, due toa large refractive index difference between the semiconductor layer andthe bonding layer. Therefore, the mesa structure of the semiconductorlayer at the center region of the micro-LED may function as a pointlight source at a focal point of the micro-lens, and thus the micro-lensmay more effectively collimate the light from the micro-LED to achievesmall emission angles. As a result, a higher collected LEE (e.g., within±18.5°) may be achieved.

The micro-LEDs described herein may be used in conjunction with varioustechnologies, such as an artificial reality system. An artificialreality system, such as a head-mounted display (HMD) or heads-up display(HUD) system, generally includes a display configured to presentartificial images that depict objects in a virtual environment. Thedisplay may present virtual objects or combine images of real objectswith virtual objects, as in virtual reality (VR), augmented reality(AR), or mixed reality (MR) applications. For example, in an AR system,a user may view both displayed images of virtual objects (e.g.,computer-generated images (CGIs)) and the surrounding environment by,for example, seeing through transparent display glasses or lenses (oftenreferred to as optical see-through) or viewing displayed images of thesurrounding environment captured by a camera (often referred to as videosee-through). In some AR systems, the artificial images may be presentedto users using an LED-based display subsystem.

In the following description, for the purposes of explanation, specificdetails are set forth in order to provide a thorough understanding ofexamples of the disclosure. However, it will be apparent that variousexamples may be practiced without these specific details. For example,devices, systems, structures, assemblies, methods, and other componentsmay be shown as components in block diagram form in order not to obscurethe examples in unnecessary detail. In other instances, well-knowndevices, processes, systems, structures, and techniques may be shownwithout necessary detail in order to avoid obscuring the examples. TheFIGS. and description are not intended to be restrictive. The terms andexpressions that have been employed in this disclosure are used as termsof description and not of limitation, and there is no intention in theuse of such terms and expressions of excluding any equivalents of thefeatures shown and described or portions thereof. The word “example” isused herein to mean “serving as an example, instance, or illustration.”Any embodiment or design described herein as “example” is notnecessarily to be construed as preferred or advantageous over otherembodiments or designs.

FIG. 1 is a simplified block diagram of an example of an artificialreality system environment 100 including a near-eye display 120 inaccordance with certain embodiments. Artificial reality systemenvironment 100 shown in FIG. 1 may include near-eye display 120, anoptional external imaging device 150, and an optional input/outputinterface 140, each of which may be coupled to an optional console 110.While FIG. 1 shows an example of artificial reality system environment100 including one near-eye display 120, one external imaging device 150,and one input/output interface 140, any number of these components maybe included in artificial reality system environment 100, or any of thecomponents may be omitted. For example, there may be multiple near-eyedisplays 120 monitored by one or more external imaging devices 150 incommunication with console 110. In some configurations, artificialreality system environment 100 may not include external imaging device150, optional input/output interface 140, and optional console 110. Inalternative configurations, different or additional components may beincluded in artificial reality system environment 100.

Near-eye display 120 may be a head-mounted display that presents contentto a user. Examples of content presented by near-eye display 120 includeone or more of images, videos, audio, or any combination thereof. Insome embodiments, audio may be presented via an external device (e.g.,speakers and/or headphones) that receives audio information fromnear-eye display 120, console 110, or both, and presents audio databased on the audio information. Near-eye display 120 may include one ormore rigid bodies, which may be rigidly or non-rigidly coupled to eachother. A rigid coupling between rigid bodies may cause the coupled rigidbodies to act as a single rigid entity. A non-rigid coupling betweenrigid bodies may allow the rigid bodies to move relative to each other.In various embodiments, near-eye display 120 may be implemented in anysuitable form-factor, including a pair of glasses. Some embodiments ofnear-eye display 120 are further described below with respect to FIGS. 2and 3 . Additionally, in various embodiments, the functionalitydescribed herein may be used in a headset that combines images of anenvironment external to near-eye display 120 and artificial realitycontent (e.g., computer-generated images). Therefore, near-eye display120 may augment images of a physical, real-world environment external tonear-eye display 120 with generated content (e.g., images, video, sound,etc.) to present an augmented reality to a user.

In various embodiments, near-eye display 120 may include one or more ofdisplay electronics 122, display optics 124, and an eye-tracking unit130. In some embodiments, near-eye display 120 may also include one ormore locators 126, one or more position sensors 128, and an inertialmeasurement unit (IMU) 132. Near-eye display 120 may omit any ofeye-tracking unit 130, locators 126, position sensors 128, and IMU 132,or include additional elements in various embodiments. Additionally, insome embodiments, near-eye display 120 may include elements combiningthe function of various elements described in conjunction with FIG. 1 .

Display electronics 122 may display or facilitate the display of imagesto the user according to data received from, for example, console 110.In various embodiments, display electronics 122 may include one or moredisplay panels, such as a liquid crystal display (LCD), an organic lightemitting diode (OLED) display, an inorganic light emitting diode (ILED)display, a micro light emitting diode (μLED) display, an active-matrixOLED display (AMOLED), a transparent OLED display (TOLED), or some otherdisplay. For example, in one implementation of near-eye display 120,display electronics 122 may include a front TOLED panel, a rear displaypanel, and an optical component (e.g., an attenuator, polarizer, ordiffractive or spectral film) between the front and rear display panels.Display electronics 122 may include pixels to emit light of apredominant color such as red, green, blue, white, or yellow. In someimplementations, display electronics 122 may display a three-dimensional(3D) image through stereoscopic effects produced by two-dimensionalpanels to create a subjective perception of image depth. For example,display electronics 122 may include a left display and a right displaypositioned in front of a user's left eye and right eye, respectively.The left and right displays may present copies of an image shiftedhorizontally relative to each other to create a stereoscopic effect(i.e., a perception of image depth by a user viewing the image).

In certain embodiments, display optics 124 may display image contentoptically (e.g., using optical waveguides and couplers) or magnify imagelight received from display electronics 122, correct optical errorsassociated with the image light, and present the corrected image lightto a user of near-eye display 120. In various embodiments, displayoptics 124 may include one or more optical elements, such as, forexample, a substrate, optical waveguides, an aperture, a Fresnel lens, aconvex lens, a concave lens, a filter, input/output couplers, or anyother suitable optical elements that may affect image light emitted fromdisplay electronics 122. Display optics 124 may include a combination ofdifferent optical elements as well as mechanical couplings to maintainrelative spacing and orientation of the optical elements in thecombination. One or more optical elements in display optics 124 may havean optical coating, such as an anti-reflective coating, a reflectivecoating, a filtering coating, or a combination of different opticalcoatings.

Locators 126 may be objects located in specific positions on near-eyedisplay 120 relative to one another and relative to a reference point onnear-eye display 120. In some implementations, console 110 may identifylocators 126 in images captured by external imaging device 150 todetermine the artificial reality headset's position, orientation, orboth. A locator 126 may be an LED, a corner cube reflector, a reflectivemarker, a type of light source that contrasts with an environment inwhich near-eye display 120 operates, or any combination thereof. Inembodiments where locators 126 are active components (e.g., LEDs orother types of light emitting devices).

External imaging device 150 may include one or more cameras, one or morevideo cameras, any other device capable of capturing images includingone or more of locators 126, or any combination thereof. Additionally,external imaging device 150 may include one or more filters (e.g., toincrease signal to noise ratio). External imaging device 150 may beconfigured to detect light emitted or reflected from locators 126 in afield of view of external imaging device 150. In embodiments wherelocators 126 include passive elements (e.g., retroreflectors), externalimaging device 150 may include a light source that illuminates some orall of locators 126, which may retro-reflect the light to the lightsource in external imaging device 150. Slow calibration data may becommunicated from external imaging device 150 to console 110, andexternal imaging device 150 may receive one or more calibrationparameters from console 110 to adjust one or more imaging parameters(e.g., focal length, focus, frame rate, sensor temperature, shutterspeed, aperture, etc.).

Position sensors 128 may generate one or more measurement signals inresponse to motion of near-eye display 120. Examples of position sensors128 may include accelerometers, gyroscopes, magnetometers, othermotion-detecting or error-correcting sensors, or any combinationthereof. For example, in some embodiments, position sensors 128 mayinclude multiple accelerometers to measure translational motion (e.g.,forward/back, up/down, or left/right) and multiple gyroscopes to measurerotational motion (e.g., pitch, yaw, or roll). In some embodiments,various position sensors may be oriented orthogonally to each other.

IMU 132 may be an electronic device that generates fast calibration databased on measurement signals received from one or more of positionsensors 128. Position sensors 128 may be located external to IMU 132,internal to IMU 132, or any combination thereof. Based on the one ormore measurement signals from one or more position sensors 128, IMU 132may generate fast calibration data indicating an estimated position ofnear-eye display 120 relative to an initial position of near-eye display120.

Eye-tracking unit 130 may include one or more eye-tracking systems. Eyetracking may refer to determining an eye's position, includingorientation and location of the eye, relative to near-eye display 120.An eye-tracking system may include an imaging system to image one ormore eyes and may optionally include a light emitter, which may generatelight that is directed to an eye such that light reflected by the eyemay be captured by the imaging system. Near-eye display 120 may use theorientation of the eye to, e.g., determine an inter-pupillary distance(IPD) of the user, determine gaze direction, introduce depth cues (e.g.,blur image outside of the user's main line of sight), collect heuristicson the user interaction in the VR media (e.g., time spent on anyparticular subject, object, or frame as a function of exposed stimuli),some other functions that are based in part on the orientation of atleast one of the user's eyes, or any combination thereof.

Input/output interface 140 may be a device that allows a user to sendaction requests to console 110. An action request may be a request toperform a particular action. For example, an action request may be tostart or to end an application or to perform a particular action withinthe application. Input/output interface 140 may include one or moreinput devices. Example input devices may include a keyboard, a mouse, agame controller, a glove, a button, a touch screen, or any othersuitable device for receiving action requests and communicating thereceived action requests to console 110. An action request received bythe input/output interface 140 may be communicated to console 110, whichmay perform an action corresponding to the requested action. In someembodiments, input/output interface 140 may provide haptic feedback tothe user in accordance with instructions received from console 110. Insome embodiments, external imaging device 150 may be used to trackinput/output interface 140, such as tracking the location or position ofa controller (which may include, for example, an IR light source) or ahand of the user to determine the motion of the user. In someembodiments, near-eye display 120 may include one or more imagingdevices to track input/output interface 140, such as tracking thelocation or position of a controller or a hand of the user to determinethe motion of the user.

Console 110 may provide content to near-eye display 120 for presentationto the user in accordance with information received from one or more ofexternal imaging device 150, near-eye display 120, and input/outputinterface 140. In the example shown in FIG. 1 , console 110 may includean application store 112, a headset tracking module 114, an artificialreality engine 116, and an eye-tracking module 118. Some embodiments ofconsole 110 may include different or additional modules than thosedescribed in conjunction with FIG. 1 . Functions further described belowmay be distributed among components of console 110 in a different mannerthan is described here.

In some embodiments, console 110 may include a processor and anon-transitory computer-readable storage medium storing instructionsexecutable by the processor. The processor may include multipleprocessing units executing instructions in parallel. The non-transitorycomputer-readable storage medium may be any memory, such as a hard diskdrive, a removable memory, or a solid-state drive (e.g., flash memory ordynamic random access memory (DRAM)). In various embodiments, themodules of console 110 described in conjunction with FIG. 1 may beencoded as instructions in the non-transitory computer-readable storagemedium that, when executed by the processor, cause the processor toperform the functions further described below.

Application store 112 may store one or more applications for executionby console 110. An application may include a group of instructions that,when executed by a processor, generates content for presentation to theuser. Content generated by an application may be in response to inputsreceived from the user via movement of the user's eyes or inputsreceived from the input/output interface 140. Examples of theapplications may include gaming applications, conferencing applications,video playback application, or other suitable applications.

Headset tracking module 114 may track movements of near-eye display 120using slow calibration information from external imaging device 150. Forexample, headset tracking module 114 may determine positions of areference point of near-eye display 120 using observed locators from theslow calibration information and a model of near-eye display 120.Headset tracking module 114 may also determine positions of a referencepoint of near-eye display 120 using position information from the fastcalibration information. Additionally, in some embodiments, headsettracking module 114 may use portions of the fast calibrationinformation, the slow calibration information, or any combinationthereof, to predict a future location of near-eye display 120. Headsettracking module 114 may provide the estimated or predicted futureposition of near-eye display 120 to artificial reality engine 116.

Artificial reality engine 116 may execute applications within artificialreality system environment 100 and receive position information ofnear-eye display 120, acceleration information of near-eye display 120,velocity information of near-eye display 120, predicted future positionsof near-eye display 120, or any combination thereof from headsettracking module 114. Artificial reality engine 116 may also receiveestimated eye position and orientation information from eye-trackingmodule 118. Based on the received information, artificial reality engine116 may determine content to provide to near-eye display 120 forpresentation to the user. Artificial reality engine 116 may perform anaction within an application executing on console 110 in response to anaction request received from input/output interface 140, and providefeedback to the user indicating that the action has been performed. Thefeedback may be visual or audible feedback via near-eye display 120 orhaptic feedback via input/output interface 140.

Eye-tracking module 118 may receive eye-tracking data from eye-trackingunit 130 and determine the position of the user's eye based on the eyetracking data. The position of the eye may include an eye's orientation,location, or both relative to near-eye display 120 or any elementthereof. Because the eye's axes of rotation change as a function of theeye's location in its socket, determining the eye's location in itssocket may allow eye-tracking module 118 to determine the eye'sorientation more accurately.

FIG. 2 is a perspective view of an example of a near-eye display in theform of an HMD device 200 for implementing some of the examplesdisclosed herein. HMD device 200 may be a part of, e.g., a VR system, anAR system, an MR system, or any combination thereof. HMD device 200 mayinclude a body 220 and a head strap 230. FIG. 2 shows a bottom side 223,a front side 225, and a left side 227 of body 220 in the perspectiveview. Head strap 230 may have an adjustable or extendible length. Theremay be a sufficient space between body 220 and head strap 230 of HMDdevice 200 for allowing a user to mount HMD device 200 onto the user'shead. In various embodiments, HMD device 200 may include additional,fewer, or different components. For example, in some embodiments, HMDdevice 200 may include eyeglass temples and temple tips as shown in, forexample, FIG. 3 below, rather than head strap 230.

HMD device 200 may present to a user media including virtual and/oraugmented views of a physical, real-world environment withcomputer-generated elements. Examples of the media presented by HMDdevice 200 may include images (e.g., two-dimensional (2D) orthree-dimensional (3D) images), videos (e.g., 2D or 3D videos), audio,or any combination thereof. The images and videos may be presented toeach eye of the user by one or more display assemblies (not shown inFIG. 2 ) enclosed in body 220 of HMD device 200. In various embodiments,the one or more display assemblies may include a single electronicdisplay panel or multiple electronic display panels (e.g., one displaypanel for each eye of the user). Examples of the electronic displaypanel(s) may include, for example, an LCD, an OLED display, an ILEDdisplay, a μLED display, an AMOLED, a TOLED, some other display, or anycombination thereof. HMD device 200 may include two eye box regions.

In some implementations, HMD device 200 may include various sensors (notshown), such as depth sensors, motion sensors, position sensors, and eyetracking sensors. Some of these sensors may use a structured lightpattern for sensing. In some implementations, HMD device 200 may includean input/output interface for communicating with a console. In someimplementations, HMD device 200 may include a virtual reality engine(not shown) that can execute applications within HMD device 200 andreceive depth information, position information, accelerationinformation, velocity information, predicted future positions, or anycombination thereof of HMD device 200 from the various sensors. In someimplementations, the information received by the virtual reality enginemay be used for producing a signal (e.g., display instructions) to theone or more display assemblies. In some implementations, HMD device 200may include locators (not shown, such as locators 126) located in fixedpositions on body 220 relative to one another and relative to areference point. Each of the locators may emit light that is detectableby an external imaging device.

FIG. 3 is a perspective view of an example of a near-eye display 300 inthe form of a pair of glasses for implementing some of the examplesdisclosed herein. Near-eye display 300 may be a specific implementationof near-eye display 120 of FIG. 1 , and may be configured to operate asa virtual reality display, an augmented reality display, and/or a mixedreality display. Near-eye display 300 may include a frame 305 and adisplay 310. Display 310 may be configured to present content to a user.In some embodiments, display 310 may include display electronics and/ordisplay optics. For example, as described above with respect to near-eyedisplay 120 of FIG. 1 , display 310 may include an LCD display panel, anLED display panel, or an optical display panel (e.g., a waveguidedisplay assembly).

Near-eye display 300 may further include various sensors 350 a, 350 b,350 c, 350 d, and 350 e on or within frame 305. In some embodiments,sensors 350 a-350 e may include one or more depth sensors, motionsensors, position sensors, inertial sensors, or ambient light sensors.In some embodiments, sensors 350 a-350 e may include one or more imagesensors configured to generate image data representing different fieldsof views in different directions. In some embodiments, sensors 350 a-350e may be used as input devices to control or influence the displayedcontent of near-eye display 300, and/or to provide an interactiveVR/AR/MR experience to a user of near-eye display 300. In someembodiments, sensors 350 a-350 e may also be used for stereoscopicimaging.

In some embodiments, near-eye display 300 may further include one ormore illuminators 330 to project light into the physical environment.The projected light may be associated with different frequency bands(e.g., visible light, infra-red light, ultra-violet light, etc.), andmay serve various purposes. For example, illuminator(s) 330 may projectlight in a dark environment (or in an environment with low intensity ofinfra-red light, ultra-violet light, etc.) to assist sensors 350 a-350 ein capturing images of different objects within the dark environment. Insome embodiments, illuminator(s) 330 may be used to project certainlight patterns onto the objects within the environment. In someembodiments, illuminator(s) 330 may be used as locators, such aslocators 126 described above with respect to FIG. 1 .

In some embodiments, near-eye display 300 may also include ahigh-resolution camera 340. Camera 340 may capture images of thephysical environment in the field of view. The captured images may beprocessed, for example, by a virtual reality engine (e.g., artificialreality engine 116 of FIG. 1 ) to add virtual objects to the capturedimages or modify physical objects in the captured images, and theprocessed images may be displayed to the user by display 310 for AR orMR applications.

FIG. 4 illustrates an example of an optical see-through augmentedreality system 400 including a waveguide display according to certainembodiments. Augmented reality system 400 may include a projector 410and a combiner 415. Projector 410 may include a light source or imagesource 412 and projector optics 414. In some embodiments, light sourceor image source 412 may include one or more micro-LED devices describedabove. In some embodiments, image source 412 may include a plurality ofpixels that displays virtual objects, such as an LCD display panel or anLED display panel. In some embodiments, image source 412 may include alight source that generates coherent or partially coherent light. Forexample, image source 412 may include a laser diode, a vertical cavitysurface emitting laser, an LED, and/or a micro-LED described above. Insome embodiments, image source 412 may include a plurality of lightsources (e.g., an array of micro-LEDs described above), each emitting amonochromatic image light corresponding to a primary color (e.g., red,green, or blue). In some embodiments, image source 412 may include threetwo-dimensional arrays of micro-LEDs, where each two-dimensional arrayof micro-LEDs may include micro-LEDs configured to emit light of aprimary color (e.g., red, green, or blue). In some embodiments, imagesource 412 may include an optical pattern generator, such as a spatiallight modulator. Projector optics 414 may include one or more opticalcomponents that can condition the light from image source 412, such asexpanding, collimating, scanning, or projecting light from image source412 to combiner 415. The one or more optical components may include, forexample, one or more lenses, liquid lenses, mirrors, apertures, and/orgratings. For example, in some embodiments, image source 412 may includeone or more one-dimensional arrays or elongated two-dimensional arraysof micro-LEDs, and projector optics 414 may include one or moreone-dimensional scanners (e.g., micro-mirrors or prisms) configured toscan the one-dimensional arrays or elongated two-dimensional arrays ofmicro-LEDs to generate image frames. In some embodiments, projectoroptics 414 may include a liquid lens (e.g., a liquid crystal lens) witha plurality of electrodes that allows scanning of the light from imagesource 412.

Combiner 415 may include an input coupler 430 for coupling light fromprojector 410 into a substrate 420 of combiner 415. Combiner 415 maytransmit at least 50% of light in a first wavelength range and reflectat least 25% of light in a second wavelength range. For example, thefirst wavelength range may be visible light from about 400 nm to about650 nm, and the second wavelength range may be in the infrared band, forexample, from about 800 nm to about 1000 nm. Input coupler 430 mayinclude a volume holographic grating, a diffractive optical element(DOE) (e.g., a surface-relief grating), a slanted surface of substrate420, or a refractive coupler (e.g., a wedge or a prism). For example,input coupler 430 may include a reflective volume Bragg grating or atransmissive volume Bragg grating. Input coupler 430 may have a couplingefficiency of greater than 30%, 50%, 75%, 90%, or higher for visiblelight. Light coupled into substrate 420 may propagate within substrate420 through, for example, total internal reflection (TIR). Substrate 420may be in the form of a lens of a pair of eyeglasses. Substrate 420 mayhave a flat or a curved surface, and may include one or more types ofdielectric materials, such as glass, quartz, plastic, polymer,poly(methyl methacrylate) (PMMA), crystal, or ceramic. A thickness ofthe substrate may range from, for example, less than about 1 mm to about10 mm or more. Substrate 420 may be transparent to visible light.

Substrate 420 may include or may be coupled to a plurality of outputcouplers 440, each configured to extract at least a portion of the lightguided by and propagating within substrate 420 from substrate 420, anddirect extracted light 460 to an eyebox 495 where an eye 490 of the userof augmented reality system 400 may be located when augmented realitysystem 400 is in use. The plurality of output couplers 440 may replicatethe exit pupil to increase the size of eyebox 495 such that thedisplayed image is visible in a larger area. As input coupler 430,output couplers 440 may include grating couplers (e.g., volumeholographic gratings or surface-relief gratings), other diffractionoptical elements (DOEs), prisms, etc. For example, output couplers 440may include reflective volume Bragg gratings or transmissive volumeBragg gratings. Output couplers 440 may have different coupling (e.g.,diffraction) efficiencies at different locations. Substrate 420 may alsoallow light 450 from the environment in front of combiner 415 to passthrough with little or no loss. Output couplers 440 may also allow light450 to pass through with little loss. For example, in someimplementations, output couplers 440 may have a very low diffractionefficiency for light 450 such that light 450 may be refracted orotherwise pass through output couplers 440 with little loss, and thusmay have a higher intensity than extracted light 460. In someimplementations, output couplers 440 may have a high diffractionefficiency for light 450 and may diffract light 450 in certain desireddirections (i.e., diffraction angles) with little loss. As a result, theuser may be able to view combined images of the environment in front ofcombiner 415 and images of virtual objects projected by projector 410.

FIG. 5A illustrates an example of a near-eye display (NED) device 500including a waveguide display 530 according to certain embodiments. NEDdevice 500 may be an example of near-eye display 120, augmented realitysystem 400, or another type of display device. NED device 500 mayinclude a light source 510, projection optics 520, and waveguide display530. Light source 510 may include multiple panels of light emitters fordifferent colors, such as a panel of red light emitters 512, a panel ofgreen light emitters 514, and a panel of blue light emitters 516. Thered light emitters 512 are organized into an array; the green lightemitters 514 are organized into an array; and the blue light emitters516 are organized into an array. The dimensions and pitches of lightemitters in light source 510 may be small. For example, each lightemitter may have a diameter less than 2 μm (e.g., about 1.2 μm) and thepitch may be less than 2 μm (e.g., about 1.5 μm). As such, the number oflight emitters in each red light emitters 512, green light emitters 514,and blue light emitters 516 can be equal to or greater than the numberof pixels in a display image, such as 960×720, 1280×720, 1440×1080,1920×1080, 2160×1080, or 2560×1080 pixels. Thus, a display image may begenerated simultaneously by light source 510. A scanning element may notbe used in NED device 500.

Before reaching waveguide display 530, the light emitted by light source510 may be conditioned by projection optics 520, which may include alens array. Projection optics 520 may collimate or focus the lightemitted by light source 510 to waveguide display 530, which may includea coupler 532 for coupling the light emitted by light source 510 intowaveguide display 530. The light coupled into waveguide display 530 maypropagate within waveguide display 530 through, for example, totalinternal reflection as described above with respect to FIG. 4 . Coupler532 may also couple portions of the light propagating within waveguidedisplay 530 out of waveguide display 530 and towards user's eye 590.

FIG. 5B illustrates an example of a near-eye display (NED) device 550including a waveguide display 580 according to certain embodiments. Insome embodiments, NED device 550 may use a scanning mirror 570 toproject light from a light source 540 to an image field where a user'seye 590 may be located. NED device 550 may be an example of near-eyedisplay 120, augmented reality system 400, or another type of displaydevice. Light source 540 may include one or more rows or one or morecolumns of light emitters of different colors, such as multiple rows ofred light emitters 542, multiple rows of green light emitters 544, andmultiple rows of blue light emitters 546. For example, red lightemitters 542, green light emitters 544, and blue light emitters 546 mayeach include N rows, each row including, for example, 2560 lightemitters (pixels). The red light emitters 542 are organized into anarray; the green light emitters 544 are organized into an array; and theblue light emitters 546 are organized into an array. In someembodiments, light source 540 may include a single line of lightemitters for each color. In some embodiments, light source 540 mayinclude multiple columns of light emitters for each of red, green, andblue colors, where each column may include, for example, 1080 lightemitters. In some embodiments, the dimensions and/or pitches of thelight emitters in light source 540 may be relatively large (e.g., about3-5 μm) and thus light source 540 may not include sufficient lightemitters for simultaneously generating a full display image. Forexample, the number of light emitters for a single color may be fewerthan the number of pixels (e.g., 2560×1080 pixels) in a display image.The light emitted by light source 540 may be a set of collimated ordiverging beams of light.

Before reaching scanning mirror 570, the light emitted by light source540 may be conditioned by various optical devices, such as collimatinglenses or a freeform optical element 560. Freeform optical element 560may include, for example, a multi-facet prism or another light foldingelement that may direct the light emitted by light source 540 towardsscanning mirror 570, such as changing the propagation direction of thelight emitted by light source 540 by, for example, about 90° or larger.In some embodiments, freeform optical element 560 may be rotatable toscan the light. Scanning mirror 570 and/or freeform optical element 560may reflect and project the light emitted by light source 540 towaveguide display 580, which may include a coupler 582 for coupling thelight emitted by light source 540 into waveguide display 580. The lightcoupled into waveguide display 580 may propagate within waveguidedisplay 580 through, for example, total internal reflection as describedabove with respect to FIG. 4 . Coupler 582 may also couple portions ofthe light propagating within waveguide display 580 out of waveguidedisplay 580 and towards user's eye 590.

Scanning mirror 570 may include a microelectromechanical system (MEMS)mirror or any other suitable mirrors. Scanning mirror 570 may rotate toscan in one or two dimensions. As scanning mirror 570 rotates, the lightemitted by light source 540 may be directed to a different area ofwaveguide display 580 such that a full display image may be projectedonto waveguide display 580 and directed to user's eye 590 by waveguidedisplay 580 in each scanning cycle. For example, in embodiments wherelight source 540 includes light emitters for all pixels in one or morerows or columns, scanning mirror 570 may be rotated in the column or rowdirection (e.g., x or y direction) to scan an image. In embodimentswhere light source 540 includes light emitters for some but not allpixels in one or more rows or columns, scanning mirror 570 may berotated in both the row and column directions (e.g., both x and ydirections) to project a display image (e.g., using a raster-typescanning pattern).

NED device 550 may operate in predefined display periods. A displayperiod (e.g., display cycle) may refer to a duration of time in which afull image is scanned or projected. For example, a display period may bea reciprocal of the desired frame rate. In NED device 550 that includesscanning mirror 570, the display period may also be referred to as ascanning period or scanning cycle. The light generation by light source540 may be synchronized with the rotation of scanning mirror 570. Forexample, each scanning cycle may include multiple scanning steps, wherelight source 540 may generate a different light pattern in eachrespective scanning step.

In each scanning cycle, as scanning mirror 570 rotates, a display imagemay be projected onto waveguide display 580 and user's eye 590. Theactual color value and light intensity (e.g., brightness) of a givenpixel location of the display image may be an average of the light beamsof the three colors (e.g., red, green, and blue) illuminating the pixellocation during the scanning period. After completing a scanning period,scanning mirror 570 may revert back to the initial position to projectlight for the first few rows of the next display image or may rotate ina reverse direction or scan pattern to project light for the nextdisplay image, where a new set of driving signals may be fed to lightsource 540. The same process may be repeated as scanning mirror 570rotates in each scanning cycle. As such, different images may beprojected to user's eye 590 in different scanning cycles.

FIG. 6 illustrates an example of an image source assembly 610 in anear-eye display system 600 according to certain embodiments. Imagesource assembly 610 may include, for example, a display panel 640 thatmay generate display images to be projected to the user's eyes, and aprojector 650 that may project the display images generated by displaypanel 640 to a waveguide display as described above with respect toFIGS. 4-5B. Display panel 640 may include a light source 642 and a drivecircuit 644 for light source 642. Light source 642 may include, forexample, light source 510 or 540. Projector 650 may include, forexample, freeform optical element 560, scanning mirror 570, and/orprojection optics 520 described above. Near-eye display system 600 mayalso include a controller 620 that synchronously controls light source642 and projector 650 (e.g., scanning mirror 570). Image source assembly610 may generate and output an image light to a waveguide display (notshown in FIG. 6 ), such as waveguide display 530 or 580. As describedabove, the waveguide display may receive the image light at one or moreinput-coupling elements, and guide the received image light to one ormore output-coupling elements. The input and output coupling elementsmay include, for example, a diffraction grating, a holographic grating,a prism, or any combination thereof. The input-coupling element may bechosen such that total internal reflection occurs with the waveguidedisplay. The output-coupling element may couple portions of the totalinternally reflected image light out of the waveguide display.

As described above, light source 642 may include a plurality of lightemitters arranged in an array or a matrix. Each light emitter may emitmonochromatic light, such as red light, blue light, green light,infra-red light, and the like. While RGB colors are often discussed inthis disclosure, embodiments described herein are not limited to usingred, green, and blue as primary colors. Other colors can also be used asthe primary colors of near-eye display system 600. In some embodiments,a display panel in accordance with an embodiment may use more than threeprimary colors. Each pixel in light source 642 may include threesubpixels that include a red micro-LED, a green micro-LED, and a bluemicro-LED. A semiconductor LED generally includes an active lightemitting layer within multiple layers of semiconductor materials. Themultiple layers of semiconductor materials may include differentcompound materials or a same base material with different dopants and/ordifferent doping densities. For example, the multiple layers ofsemiconductor materials may include an n-type material layer, an activeregion that may include hetero-structures (e.g., one or more quantumwells), and a p-type material layer. The multiple layers ofsemiconductor materials may be grown on a surface of a substrate havinga certain orientation. In some embodiments, to increase light extractionefficiency, a mesa that includes at least some of the layers ofsemiconductor materials may be formed.

Controller 620 may control the image rendering operations of imagesource assembly 610, such as the operations of light source 642 and/orprojector 650. For example, controller 620 may determine instructionsfor image source assembly 610 to render one or more display images. Theinstructions may include display instructions and scanning instructions.In some embodiments, the display instructions may include an image file(e.g., a bitmap file). The display instructions may be received from,for example, a console, such as console 110 described above with respectto FIG. 1 . The scanning instructions may be used by image sourceassembly 610 to generate image light. The scanning instructions mayspecify, for example, a type of a source of image light (e.g.,monochromatic or polychromatic), a scanning rate, an orientation of ascanning apparatus, one or more illumination parameters, or anycombination thereof. Controller 620 may include a combination ofhardware, software, and/or firmware not shown here so as not to obscureother aspects of the present disclosure.

In some embodiments, controller 620 may be a graphics processing unit(GPU) of a display device. In other embodiments, controller 620 may beother kinds of processors. The operations performed by controller 620may include taking content for display and dividing the content intodiscrete sections. Controller 620 may provide to light source 642scanning instructions that include an address corresponding to anindividual source element of light source 642 and/or an electrical biasapplied to the individual source element. Controller 620 may instructlight source 642 to sequentially present the discrete sections usinglight emitters corresponding to one or more rows of pixels in an imageultimately displayed to the user. Controller 620 may also instructprojector 650 to perform different adjustments of the light. Forexample, controller 620 may control projector 650 to scan the discretesections to different areas of a coupling element of the waveguidedisplay (e.g., waveguide display 580) as described above with respect toFIG. 5B. As such, at the exit pupil of the waveguide display, eachdiscrete portion is presented in a different respective location. Whileeach discrete section is presented at a different respective time, thepresentation and scanning of the discrete sections occur fast enoughsuch that a user's eye may integrate the different sections into asingle image or series of images.

Image processor 630 may be a general-purpose processor and/or one ormore application-specific circuits that are dedicated to performing thefeatures described herein. In one embodiment, a general-purposeprocessor may be coupled to a memory to execute software instructionsthat cause the processor to perform certain processes described herein.In another embodiment, image processor 630 may be one or more circuitsthat are dedicated to performing certain features. While image processor630 in FIG. 6 is shown as a stand-alone unit that is separate fromcontroller 620 and drive circuit 644, image processor 630 may be asub-unit of controller 620 or drive circuit 644 in other embodiments. Inother words, in those embodiments, controller 620 or drive circuit 644may perform various image processing functions of image processor 630.Image processor 630 may also be referred to as an image processingcircuit.

In the example shown in FIG. 6 , light source 642 may be driven by drivecircuit 644, based on data or instructions (e.g., display and scanninginstructions) sent from controller 620 or image processor 630. In oneembodiment, drive circuit 644 may include a circuit panel that connectsto and mechanically holds various light emitters of light source 642.Light source 642 may emit light in accordance with one or moreillumination parameters that are set by the controller 620 andpotentially adjusted by image processor 630 and drive circuit 644. Anillumination parameter may be used by light source 642 to generatelight. An illumination parameter may include, for example, sourcewavelength, pulse rate, pulse amplitude, beam type (continuous orpulsed), other parameter(s) that may affect the emitted light, or anycombination thereof. In some embodiments, the source light generated bylight source 642 may include multiple beams of red light, green light,and blue light, or any combination thereof.

Projector 650 may perform a set of optical functions, such as focusing,combining, conditioning, or scanning the image light generated by lightsource 642. In some embodiments, projector 650 may include a combiningassembly, a light conditioning assembly, or a scanning mirror assembly.Projector 650 may include one or more optical components that opticallyadjust and potentially re-direct the light from light source 642. Oneexample of the adjustment of light may include conditioning the light,such as expanding, collimating, correcting for one or more opticalerrors (e.g., field curvature, chromatic aberration, etc.), some otheradjustments of the light, or any combination thereof. The opticalcomponents of projector 650 may include, for example, lenses, mirrors,apertures, gratings, or any combination thereof.

Projector 650 may redirect image light via its one or more reflectiveand/or refractive portions so that the image light is projected atcertain orientations toward the waveguide display. The location wherethe image light is redirected toward the waveguide display may depend onspecific orientations of the one or more reflective and/or refractiveportions. In some embodiments, projector 650 includes a single scanningmirror that scans in at least two dimensions. In other embodiments,projector 650 may include a plurality of scanning mirrors that each scanin directions orthogonal to each other. Projector 650 may perform araster scan (horizontally or vertically), a bi-resonant scan, or anycombination thereof. In some embodiments, projector 650 may perform acontrolled vibration along the horizontal and/or vertical directionswith a specific frequency of oscillation to scan along two dimensionsand generate a two-dimensional projected image of the media presented touser's eyes. In other embodiments, projector 650 may include a lens orprism that may serve similar or the same function as one or morescanning mirrors. In some embodiments, image source assembly 610 may notinclude a projector, where the light emitted by light source 642 may bedirectly incident on the waveguide display.

In semiconductor LEDs, photons are usually generated at a certaininternal quantum efficiency through the recombination of electrons andholes within an active region (e.g., one or more semiconductor layers),where the internal quantum efficiency is the proportion of the radiativeelectron-hole recombination in the active region that emits photons. Thegenerated light may then be extracted from the LEDs in a particulardirection or within a particular solid angle. The ratio between thenumber of emitted photons extracted from an LED and the number ofelectrons passing through the LED is referred to as the external quantumefficiency, which describes how efficiently the LED converts injectedelectrons to photons that are extracted from the device.

The external quantum efficiency may be proportional to the injectionefficiency, the internal quantum efficiency, and the extractionefficiency. The injection efficiency refers to the proportion ofelectrons passing through the device that are injected into the activeregion. The extraction efficiency is the proportion of photons generatedin the active region that escape from the device. For LEDs, and inparticular, micro-LEDs with reduced physical dimensions, improving theinternal and external quantum efficiency and/or controlling the emissionspectrum may be challenging. In some embodiments, to increase the lightextraction efficiency, a mesa that includes at least some of the layersof semiconductor materials may be formed.

FIG. 7A illustrates an example of an LED 700 having a vertical mesastructure. LED 700 may be a light emitter in light source 510, 540, or642. LED 700 may be a micro-LED made of inorganic materials, such asmultiple layers of semiconductor materials. The layered semiconductorlight emitting device may include multiple layers of III-V semiconductormaterials. A III-V semiconductor material may include one or more GroupIII elements, such as aluminum (Al), gallium (Ga), or indium (In), incombination with a Group V element, such as nitrogen (N), phosphorus(P), arsenic (As), or antimony (Sb). When the Group V element of theIII-V semiconductor material includes nitrogen, the III-V semiconductormaterial is referred to as a III-nitride material. The layeredsemiconductor light emitting device may be manufactured by growingmultiple epitaxial layers on a substrate using techniques such asvapor-phase epitaxy (VPE), liquid-phase epitaxy (LPE), molecular beamepitaxy (MBE), or metalorganic chemical vapor deposition (MOCVD). Forexample, the layers of the semiconductor materials may be grownlayer-by-layer on a substrate with a certain crystal lattice orientation(e.g., polar, nonpolar, or semi-polar orientation), such as a GaN, GaAs,or GaP substrate, or a substrate including, but not limited to,sapphire, silicon carbide, silicon, zinc oxide, boron nitride, lithiumaluminate, lithium niobate, germanium, aluminum nitride, lithiumgallate, partially substituted spinels, or quaternary tetragonal oxidessharing the beta-LiAlO₂ structure, where the substrate may be cut in aspecific direction to expose a specific plane as the growth surface.

In the example shown in FIG. 7A, LED 700 may include a substrate 710,which may include, for example, a sapphire substrate or a GaN substrate.A semiconductor layer 720 may be grown on substrate 710. Semiconductorlayer 720 may include a III-V material, such as GaN, and may be p-doped(e.g., with Mg, Ca, Zn, or Be) or n-doped (e.g., with Si or Ge). One ormore active layers 730 may be grown on semiconductor layer 720 to forman active region. Active layer 730 may include III-V materials, such asone or more InGaN layers, one or more AlInGaP layers, and/or one or moreGaN layers, which may form one or more heterostructures, such as one ormore quantum wells or MQWs. A semiconductor layer 740 may be grown onactive layer 730. Semiconductor layer 740 may include a III-V material,such as GaN, and may be p-doped (e.g., with Mg, Ca, Zn, or Be) orn-doped (e.g., with Si or Ge). One of semiconductor layer 720 andsemiconductor layer 740 may be a p-type layer and the other one may bean n-type layer. Semiconductor layer 720 and semiconductor layer 740sandwich active layer 730 to form the light emitting region. Forexample, LED 700 may include a layer of InGaN situated between a layerof p-type GaN doped with magnesium and a layer of n-type GaN doped withsilicon or oxygen. In some embodiments, LED 700 may include a layer ofAlInGaP situated between a layer of p-type AlInGaP doped with zinc ormagnesium and a layer of n-type AlInGaP doped with selenium, silicon, ortellurium.

In some embodiments, an electron-blocking layer (EBL) (not shown in FIG.7A) may be grown to form a layer between active layer 730 and at leastone of semiconductor layer 720 or semiconductor layer 740. The EBL mayreduce the electron leakage current and improve the efficiency of theLED. In some embodiments, a heavily-doped semiconductor layer 750, suchas a P⁺ or P⁺⁺ semiconductor layer, may be formed on semiconductor layer740 and act as a contact layer for forming an ohmic contact and reducingthe contact impedance of the device. In some embodiments, a conductivelayer 760 may be formed on heavily-doped semiconductor layer 750.Conductive layer 760 may include, for example, an indium tin oxide (ITO)or Al/Ni/Au film. In one example, conductive layer 760 may include atransparent ITO layer.

To make contact with semiconductor layer 720 (e.g., an n-GaN layer) andto more efficiently extract light emitted by active layer 730 from LED700, the semiconductor material layers (including heavily-dopedsemiconductor layer 750, semiconductor layer 740, active layer 730, andsemiconductor layer 720) may be etched to expose semiconductor layer 720and to form a mesa structure that includes layers 720-760. The mesastructure may confine the carriers within the device. Etching the mesastructure may lead to the formation of mesa sidewalls 732 that may beorthogonal to the growth planes. A passivation layer 770 may be formedon mesa sidewalls 732 of the mesa structure. Passivation layer 770 mayinclude an oxide layer, such as a SiO₂ layer, and may act as a reflectorto reflect emitted light out of LED 700. A contact layer 780, which mayinclude a metal layer, such as Al, Au, Ni, Ti, or any combinationthereof, may be formed on semiconductor layer 720 and may act as anelectrode of LED 700. In addition, another contact layer 790, such as anAl/Ni/Au metal layer, may be formed on conductive layer 760 and may actas another electrode of LED 700.

When a voltage signal is applied to contact layers 780 and 790,electrons and holes may recombine in active layer 730, where therecombination of electrons and holes may cause photon emission. Thewavelength and energy of the emitted photons may depend on the energybandgap between the valence band and the conduction band in active layer730. For example, InGaN active layers may emit green or blue light,AlGaN active layers may emit blue to ultraviolet light, while AlInGaPactive layers may emit red, orange, yellow, or green light. The emittedphotons may be reflected by passivation layer 770 and may exit LED 700from the top (e.g., conductive layer 760 and contact layer 790) orbottom (e.g., substrate 710).

In some embodiments, LED 700 may include one or more other components,such as a lens, on the light emission surface, such as substrate 710, tofocus or collimate the emitted light or couple the emitted light into awaveguide. In some embodiments, an LED may include a mesa of anothershape, such as planar, conical, semi-parabolic, or parabolic, and a basearea of the mesa may be circular, rectangular, hexagonal, or triangular.For example, the LED may include a mesa of a curved shape (e.g.,paraboloid shape) and/or a non-curved shape (e.g., conic shape). Themesa may be truncated or non-truncated.

FIG. 7B is a cross-sectional view of an example of an LED 705 having aparabolic mesa structure. Similar to LED 700, LED 705 may includemultiple layers of semiconductor materials, such as multiple layers ofIII-V semiconductor materials. The semiconductor material layers may beepitaxially grown on a substrate 715, such as a GaN substrate or asapphire substrate. For example, a semiconductor layer 725 may be grownon substrate 715. Semiconductor layer 725 may include a III-V material,such as GaN, and may be p-doped (e.g., with Mg, Ca, Zn, or Be) orn-doped (e.g., with Si or Ge). One or more active layer 735 may be grownon semiconductor layer 725. Active layer 735 may include III-Vmaterials, such as one or more InGaN layers, one or more AlInGaP layers,and/or one or more GaN layers, which may form one or moreheterostructures, such as one or more quantum wells. A semiconductorlayer 745 may be grown on active layer 735. Semiconductor layer 745 mayinclude a III-V material, such as GaN, and may be p-doped (e.g., withMg, Ca, Zn, or Be) or n-doped (e.g., with Si or Ge). One ofsemiconductor layer 725 and semiconductor layer 745 may be a p-typelayer and the other one may be an n-type layer.

To make contact with semiconductor layer 725 (e.g., an n-type GaN layer)and to more efficiently extract light emitted by active layer 735 fromLED 705, the semiconductor layers may be etched to expose semiconductorlayer 725 and to form a mesa structure that includes layers 725-745. Themesa structure may confine carriers within the injection area of thedevice. Etching the mesa structure may lead to the formation of mesaside walls (also referred to herein as facets) that may be non-parallelwith, or in some cases, orthogonal, to the growth planes associated withcrystalline growth of layers 725-745.

As shown in FIG. 7B, LED 705 may have a mesa structure that includes aflat top. A dielectric layer 775 (e.g., SiO₂ or SiN) may be formed onthe facets of the mesa structure. In some embodiments, dielectric layer775 may include multiple layers of dielectric materials. In someembodiments, a metal layer 795 may be formed on dielectric layer 775.Metal layer 795 may include one or more metal or metal alloy materials,such as aluminum (Al), silver (Ag), gold (Au), platinum (Pt), titanium(Ti), copper (Cu), or any combination thereof. Dielectric layer 775 andmetal layer 795 may form a mesa reflector that can reflect light emittedby active layer 735 toward substrate 715. In some embodiments, the mesareflector may be parabolic-shaped to act as a parabolic reflector thatmay at least partially collimate the emitted light.

Electrical contact 765 and electrical contact 785 may be formed onsemiconductor layer 745 and semiconductor layer 725, respectively, toact as electrodes. Electrical contact 765 and electrical contact 785 mayeach include a conductive material, such as Al, Au, Pt, Ag, Ni, Ti, Cu,or any combination thereof (e.g., Ag/Pt/Au or Al/Ni/Au), and may act asthe electrodes of LED 705. In the example shown in FIG. 7B, electricalcontact 785 may be an n-contact, and electrical contact 765 may be ap-contact. Electrical contact 765 and semiconductor layer 745 (e.g., ap-type semiconductor layer) may form a back reflector for reflectinglight emitted by active layer 735 back toward substrate 715. In someembodiments, electrical contact 765 and metal layer 795 include samematerial(s) and can be formed using the same processes. In someembodiments, an additional conductive layer (not shown) may be includedas an intermediate conductive layer between the electrical contacts 765and 785 and the semiconductor layers.

When a voltage signal is applied across electrical contacts 765 and 785,electrons and holes may recombine in active layer 735. The recombinationof electrons and holes may cause photon emission, thus producing light.The wavelength and energy of the emitted photons may depend on theenergy bandgap between the valence band and the conduction band inactive layer 735. For example, InGaN active layers may emit green orblue light, while AlInGaP active layers may emit red, orange, yellow, orgreen light. The emitted photons may propagate in many differentdirections, and may be reflected by the mesa reflector and/or the backreflector and may exit LED 705, for example, from the bottom side (e.g.,substrate 715) shown in FIG. 7B. One or more other secondary opticalcomponents, such as a lens or a grating, may be formed on the lightemission surface, such as substrate 715, to focus or collimate theemitted light and/or couple the emitted light into a waveguide.

One or two-dimensional arrays of the LEDs described above may bemanufactured on a wafer to form light sources (e.g., light source 642).Drive circuits (e.g., drive circuit 644) may be fabricated, for example,on a silicon wafer using CMOS processes. The LEDs and the drive circuitson wafers may be diced and then bonded together, or may be bonded on thewafer level and then diced. Various bonding techniques can be used forbonding the LEDs and the drive circuits, such as adhesive bonding,metal-to-metal bonding, metal oxide bonding, wafer-to-wafer bonding,die-to-wafer bonding, hybrid bonding, and the like.

FIGS. 8A-8D illustrate an example of a method of hybrid bonding forarrays of LEDs according to certain embodiments. The hybrid bonding maygenerally include wafer cleaning and activation, high-precisionalignment of contacts of one wafer with contacts of another wafer,dielectric bonding of dielectric materials at the surfaces of the wafersat room temperature, and metal bonding of the contacts by annealing atelevated temperatures. FIG. 8A shows a substrate 810 with passive oractive circuits 820 manufactured thereon. As described above withrespect to FIGS. 8A-8B, substrate 810 may include, for example, asilicon wafer. Circuits 820 may include drive circuits for the arrays ofLEDs. A bonding layer may include dielectric regions 840 and contactpads 830 connected to circuits 820 through electrical interconnects 822.Contact pads 830 may include, for example, Cu, Ag, Au, Al, W, Mo, Ni,Ti, Pt, Pd, or the like. Dielectric materials in dielectric regions 840may include SiCN, SiO₂, SiN, Al₂O₃, HfO₂, ZrO₂, Ta₂O₅, or the like. Thebonding layer may be planarized and polished using, for example,chemical mechanical polishing, where the planarization or polishing maycause dishing (a bowl like profile) in the contact pads. The surfaces ofthe bonding layers may be cleaned and activated by, for example, an ion(e.g., plasma) or fast atom (e.g., Ar) beam 805. The activated surfacemay be atomically clean and may be reactive for formation of directbonds between wafers when they are brought into contact, for example, atroom temperature.

FIG. 8B illustrates a wafer 850 including an array of micro-LEDs 870fabricated thereon as described above with respect to, for example,FIGS. 7A-8B. Wafer 850 may be a carrier wafer and may include, forexample, GaAs, InP, GaN, AlN, sapphire, SiC, Si, or the like. Micro-LEDs870 may include an n-type layer, an active region, and a p-type layerepitaxially grown on wafer 850. The epitaxial layers may include variousIII-V semiconductor materials described above, and may be processed fromthe p-type layer side to etch mesa structures in the epitaxial layers,such as substantially vertical structures, parabolic structures, conicstructures, or the like. Passivation layers and/or reflection layers maybe formed on the sidewalls of the mesa structures. P-contacts 880 andn-contacts 882 may be formed in a dielectric material layer 860deposited on the mesa structures and may make electrical contacts withthe p-type layer and the n-type layers, respectively. Dielectricmaterials in dielectric material layer 860 may include, for example,SiCN, SiO₂, SiN, Al₂O₃, HfO₂, ZrO₂, Ta₂O₅, or the like. P-contacts 880and n-contacts 882 may include, for example, Cu, Ag, Au, Al, W, Mo, Ni,Ti, Pt, Pd, or the like. The top surfaces of p-contacts 880, n-contacts882, and dielectric material layer 860 may form a bonding layer. Thebonding layer may be planarized and polished using, for example,chemical mechanical polishing, where the polishing may cause dishing inp-contacts 880 and n-contacts 882. The bonding layer may then be cleanedand activated by, for example, an ion (e.g., plasma) or fast atom (e.g.,Ar) beam 815. The activated surface may be atomically clean and reactivefor formation of direct bonds between wafers when they are brought intocontact, for example, at room temperature.

FIG. 8C illustrates a room temperature bonding process for bonding thedielectric materials in the bonding layers. For example, after thebonding layer that includes dielectric regions 840 and contact pads 830and the bonding layer that includes p-contacts 880, n-contacts 882, anddielectric material layer 860 are surface activated, wafer 850 andmicro-LEDs 870 may be turned upside down and brought into contact withsubstrate 810 and the circuits formed thereon. In some embodiments,compression pressure 825 may be applied to substrate 810 and wafer 850such that the bonding layers are pressed against each other. Due to thesurface activation and the dishing in the contacts, dielectric regions840 and dielectric material layer 860 may be in direct contact becauseof the surface attractive force, and may react and form chemical bondsbetween them because the surface atoms may have dangling bonds and maybe in unstable energy states after the activation. Thus, the dielectricmaterials in dielectric regions 840 and dielectric material layer 860may be bonded together with or without heat treatment or pressure.

FIG. 8D illustrates an annealing process for bonding the contacts in thebonding layers after bonding the dielectric materials in the bondinglayers. For example, contact pads 830 and p-contacts 880 or n-contacts882 may be bonded together by annealing at, for example, about 200-400°C. or higher. During the annealing process, heat 835 may cause thecontacts to expand more than the dielectric materials (due to differentcoefficients of thermal expansion), and thus may close the dishing gapsbetween the contacts such that contact pads 830 and p-contacts 880 orn-contacts 882 may be in contact and may form direct metallic bonds atthe activated surfaces.

In some embodiments where the two bonded wafers include materials havingdifferent coefficients of thermal expansion (CTEs), the dielectricmaterials bonded at room temperature may help to reduce or preventmisalignment of the contact pads caused by the different thermalexpansions. In some embodiments, to further reduce or avoid themisalignment of the contact pads at a high temperature during annealing,trenches may be formed between micro-LEDs, between groups of micro-LEDs,through part or all of the substrate, or the like, before bonding.

After the micro-LEDs are bonded to the drive circuits, the substrate onwhich the micro-LEDs are fabricated may be thinned or removed, andvarious secondary optical components may be fabricated on the lightemitting surfaces of the micro-LEDs to, for example, extract, collimate,and redirect the light emitted from the active regions of themicro-LEDs. In one example, micro-lenses may be formed on themicro-LEDs, where each micro-lens may correspond to a respectivemicro-LED and may help to improve the light extraction efficiency andcollimate the light emitted by the micro-LED. In some embodiments, thesecondary optical components may be fabricated in the substrate or then-type layer of the micro-LEDs. In some embodiments, the secondaryoptical components may be fabricated in a dielectric layer deposited onthe n-type side of the micro-LEDs. Examples of the secondary opticalcomponents may include a lens, a grating, an antireflection (AR)coating, a prism, a photonic crystal, or the like.

FIG. 9 illustrates an example of an LED array 900 with secondary opticalcomponents fabricated thereon according to certain embodiments. LEDarray 900 may be made by bonding an LED chip or wafer with a siliconwafer including electrical circuits fabricated thereon, using anysuitable bonding techniques described above with respect to, forexample, FIGS. 8A-8D. In the example shown in FIG. 9 , LED array 900 maybe bonded using a wafer-to-wafer hybrid bonding technique as describedabove with respect to FIG. 8A-8D. LED array 900 may include a substrate910, which may be, for example, a silicon wafer. Integrated circuits920, such as LED drive circuits, may be fabricated on substrate 910.Integrated circuits 920 may be connected to p-contacts 974 andn-contacts 972 of micro-LEDs 970 through interconnects 922 and contactpads 930, where contact pads 930 may form metallic bonds with p-contacts974 and n-contacts 972. Dielectric layer 940 on substrate 910 may bebonded to dielectric layer 960 through fusion bonding.

The substrate (not shown) of the LED chip or wafer may be thinned or maybe removed to expose the n-type layer 950 of micro-LEDs 970. Varioussecondary optical components, such as a spherical micro-lens 982, agrating 984, a micro-lens 986, an antireflection layer 988, and thelike, may be formed in or on top of n-type layer 950. For example,spherical micro-lens arrays may be etched in the semiconductor materialsof micro-LEDs 970 using a gray-scale mask and a photoresist with alinear response to exposure light, or using an etch mask formed bythermal reflowing of a patterned photoresist layer. The secondaryoptical components may also be etched in a dielectric layer deposited onn-type layer 950 using similar photolithographic techniques or othertechniques. For example, micro-lens arrays may be formed in a polymerlayer through thermal reflowing of the polymer layer that is patternedusing a binary mask. The micro-lens arrays in the polymer layer may beused as the secondary optical components or may be used as the etch maskfor transferring the profiles of the micro-lens arrays into a dielectriclayer or a semiconductor layer. The dielectric layer may include, forexample, SiCN, SiO₂, SiN, Al₂O₃, HfO₂, ZrO₂, Ta₂O₅, or the like. In someembodiments, a micro-LED 970 may have multiple corresponding secondaryoptical components, such as a micro-lens and an anti-reflection coating,a micro-lens etched in the semiconductor material and a micro-lensetched in a dielectric material layer, a micro-lens and a grating, aspherical lens and an aspherical lens, and the like. Three differentsecondary optical components are illustrated in FIG. 9 to show someexamples of secondary optical components that can be formed onmicro-LEDs 970, which does not necessary imply that different secondaryoptical components are used simultaneously for every LED array.

FIG. 10A illustrates an example of a method of die-to-wafer bonding forarrays of LEDs according to certain embodiments. In the example shown inFIG. 10A, an LED array 1001 may include a plurality of LEDs 1007 on acarrier substrate 1005. Carrier substrate 1005 may include variousmaterials, such as GaAs, InP, GaN, AlN, sapphire, SiC, Si, or the like.LEDs 1007 may be fabricated by, for example, growing various epitaxiallayers, forming mesa structures, and forming electrical contacts orelectrodes, before performing the bonding. The epitaxial layers mayinclude various materials, such as GaN, InGaN, (AlGaIn)P, (AlGaIn)AsP,(AlGaIn)AsN, (Eu:InGa)N, (AlGaIn)N, or the like, and may include ann-type layer, a p-type layer, and an active layer that includes one ormore heterostructures, such as one or more quantum wells or MQWs. Theelectrical contacts may include various conductive materials, such as ametal or a metal alloy.

A wafer 1003 may include a base layer 1009 having passive or activeintegrated circuits (e.g., drive circuits 1011) fabricated thereon. Baselayer 1009 may include, for example, a silicon wafer. Drive circuits1011 may be used to control the operations of LEDs 1007. For example,the drive circuit for each LED 1007 may include a 2T1C pixel structurethat has two transistors and one capacitor. Wafer 1003 may also includea bonding layer 1013. Bonding layer 1013 may include various materials,such as a metal, an oxide, a dielectric, CuSn, AuTi, and the like. Insome embodiments, a patterned layer 1015 may be formed on a surface ofbonding layer 1013, where patterned layer 1015 may include a metallicgrid made of a conductive material, such as Cu, Ag, Au, Al, or the like.

LED array 1001 may be bonded to wafer 1003 via bonding layer 1013 orpatterned layer 1015. For example, patterned layer 1015 may includemetal pads or bumps made of various materials, such as CuSn, AuSn, ornanoporous Au, that may be used to align LEDs 1007 of LED array 1001with corresponding drive circuits 1011 on wafer 1003. In one example,LED array 1001 may be brought toward wafer 1003 until LEDs 1007 comeinto contact with respective metal pads or bumps corresponding to drivecircuits 1011. Some or all of LEDs 1007 may be aligned with drivecircuits 1011, and may then be bonded to wafer 1003 via patterned layer1015 by various bonding techniques, such as metal-to-metal bonding.After LEDs 1007 have been bonded to wafer 1003, carrier substrate 1005may be removed from LEDs 1007.

For high-resolution micro-LED display panel, due to the small pitches ofthe micro-LED array and the small dimensions of individual micro-LEDs,it can be challenging to electrically connect the drive circuits to theelectrodes of the LEDs. For example, in the face-to-face bondingtechniques describe above, it is difficult to precisely align thebonding pads on the micro-LED devices with the bonding pads on the drivecircuits and form reliable bonding at the interfaces that may includeboth dielectric materials (e.g., SiO₂, SiN, or SiCN) and metal (e.g.,Cu, Au, or Al) bonding pads. In particular, when the pitch of themicro-LED device is about 2 or 3 microns or lower, the bonding pads mayhave a linear dimension less than about 1 μm in order to avoid shortingto adjacent micro-LEDs and to improve bonding strength for thedielectric bonding. However, small bonding pads may be less tolerant tomisalignments between the bonding pads, which may reduce the metalbonding area, increase the contact resistance (or may even be an opencircuit), and/or cause diffusion of metals to the dielectric materialsand the semiconductor materials. Thus, precise alignment of the bondingpads on surfaces of the micro-LED arrays and bonding pads on surfaces ofCMOS backplane may be needed in the conventional processes. However, theaccuracy of die-to-wafer or wafer-to-wafer bonding alignment usingstate-of-art equipment may be on the order of about 0.5 μm or about 1μm, which may not be adequate for bonding the small-pitch micro-LEDarrays (e.g., with a linear dimension of the bonding pads on the orderof 1 μm or shorter) to CMOS drive circuits.

In some implementations, to avoid precise alignment for the bonding, amicro-LED wafer may be bonded to a CMOS backplane after the epitaxiallayer growth and before the formation of individual micro-LED on themicro-LED wafer, where the micro-LED wafer and the CMOS backplane may bebonded through metal-to-metal bonding of two solid metal bonding layerson the two wafers. No alignment would be needed to bond the solidcontiguous metal bonding layers. After the bonding, the epitaxial layerson the micro-LED wafer and the metal bonding layers may be etched toform individual micro-LEDs. The etching process may have much higheralignment accuracy and thus may form individual micro-LEDs that alignwith the underlying pixel drive circuits.

FIG. 10B illustrates an example of a method of wafer-to-wafer bondingfor arrays of LEDs according to certain embodiments. As shown in FIG.10B, a first wafer 1002 may include a substrate 1004, a firstsemiconductor layer 1006, active layers 1008, and a second semiconductorlayer 1010. Substrate 1004 may include various materials, such as GaAs,InP, GaN, MN, sapphire, SiC, Si, or the like. First semiconductor layer1006, active layers 1008, and second semiconductor layer 1010 mayinclude various semiconductor materials, such as GaN, InGaN, (AlGaIn)P,(AlGaIn)AsP, (AlGaIn)AsN, (AlGaIn)Pas, (Eu:InGa)N, (AlGaIn)N, or thelike. In some embodiments, first semiconductor layer 1006 may be ann-type layer, and second semiconductor layer 1010 may be a p-type layer.For example, first semiconductor layer 1006 may be an n-doped GaN layer(e.g., doped with Si or Ge), and second semiconductor layer 1010 may bea p-doped GaN layer (e.g., doped with Mg, Ca, Zn, or Be). Active layers1008 may include, for example, one or more GaN layers, one or more InGaNlayers, one or more AlInGaP layers, and the like, which may form one ormore heterostructures, such as one or more quantum wells or MQWs.

In some embodiments, first wafer 1002 may also include a bonding layer.Bonding layer 1012 may include various materials, such as a metal, anoxide, a dielectric, CuSn, AuTi, or the like. In one example, bondinglayer 1012 may include p-contacts and/or n-contacts (not shown). In someembodiments, other layers may also be included on first wafer 1002, suchas a buffer layer between substrate 1004 and first semiconductor layer1006. The buffer layer may include various materials, such aspolycrystalline GaN or AlN. In some embodiments, a contact layer may bebetween second semiconductor layer 1010 and bonding layer 1012. Thecontact layer may include any suitable material for providing anelectrical contact to second semiconductor layer 1010 and/or firstsemiconductor layer 1006.

First wafer 1002 may be bonded to wafer 1003 that includes drivecircuits 1011 and bonding layer 1013 as described above, via bondinglayer 1013 and/or bonding layer 1012. Bonding layer 1012 and bondinglayer 1013 may be made of the same material or different materials.Bonding layer 1013 and bonding layer 1012 may be substantially flat.First wafer 1002 may be bonded to wafer 1003 by various methods, such asmetal-to-metal bonding, eutectic bonding, metal oxide bonding, anodicbonding, thermo-compression bonding, ultraviolet (UV) bonding, and/orfusion bonding.

As shown in FIG. 10B, first wafer 1002 may be bonded to wafer 1003 withthe p-side (e.g., second semiconductor layer 1010) of first wafer 1002facing down (i.e., toward wafer 1003). After bonding, substrate 1004 maybe removed from first wafer 1002, and first wafer 1002 may then beprocessed from the n-side. The processing may include, for example, theformation of certain mesa shapes for individual LEDs, as well as theformation of optical components corresponding to the individual LEDs.

FIGS. 11A-11F illustrate an example of a method of fabricating amicro-LED device using alignment-free metal-to-metal bonding andpost-bonding mesa formation processes. FIG. 11A shows a micro-LED wafer1102 including epitaxial layers grown on a substrate 1110. As describedabove, substrate 1110 may include, for example, a GaN, GaAs, or GaPsubstrate, or a substrate including, but not limited to, sapphire,silicon carbide, silicon, zinc oxide, boron nitride, lithium aluminate,lithium niobate, germanium, aluminum nitride, lithium gallate, partiallysubstituted spinels, or quaternary tetragonal oxides sharing thebeta-LiAlO₂ structure, where the substrate may be cut in a specificdirection to expose a specific plane (e.g., a c-plane or a semipolarplane) as the growth surface. In some embodiments, a buffer layer 1112may be formed on substrate 1110 to improve the lattice matching betweenthe growth substrate and the epitaxial layers, thereby reducing stressand defects in the epitaxial layers. The epitaxial layers may include ann-type semiconductor layer 1114 (e.g., a GaN layer doped with Si or Ge),an active region 1116, and a p-type semiconductor layer 1118 (e.g., aGaN layer doped with Mg, Ca, Zn, or Be). Active region 1116 may includemultiple quantum wells or an MQW formed by quantum well layers (e.g.,InGaN layer) sandwiched by barrier layers (e.g., GaN layer) as describedabove. The epitaxial layers may be grown layer-by-layer on substrate1110 or buffer layer 1112 using techniques such as VPE, LPE, MBE, orMOCVD.

In the epitaxial growth processes, dopants (e.g., Mg) used to dope thep-type semiconductor layer (e.g., Mg-doped GaN layer) may remain in thereactor and/or on the epitaxial surface after the introduction of Mgprecursors into the reactor. For example, the source for Mg doping(e.g., bis(cyclopentadienyl) magnesium (Cp₂Mg)) may be adsorbed ontoreactor lines and walls and may be released in the gas phase insubsequent processes. A surface riding effect can also contribute to theresidual Mg due to a Mg-rich layer formed on the surface of the p-GaNlayer. Thus, if the quantum-well layers are grown on the Mg-rich p-GaNlayer after the growth of the p-GaN layer with Mg dopants, thequantum-well layers may be contaminated with Mg dopants even after theMg source is turned off, which may be referred to as the Mg-memoryeffect and may manifest as a slow decay tail of Mg into subsequentepitaxial layers. Mg can contaminate the MQW layers to formnon-radiative recombination centers caused by, for example, Mg-relatedpoint defects, Mg interstitials, or Mg-related complexes.

In addition, for p-type GaN layers formed using, for example, MOCVD, thedopants (e.g., Mg) may be passivated due to the incorporation of atomichydrogen (which exists in the form of H⁺) during growth and theformation of Mg—H complexes. Therefore, a post-growth activation of thedopants is generally performed to release mobile holes. The activationof the dopants in the p-GaN layer may include breaking the Mg—H bondsand driving the H⁺ out of the p-GaN layer at elevated temperatures(e.g., above 700° C.) to activate the Mg dopants. Insufficientactivation of the Mg dopants in the p-GaN layer may lead to an opencircuit, a poor performance, or a premature punch-through breakdown ofthe LED device. If p-type GaN layer is grown before the growth of theactive region and the n-type layer, to drive out hydrogen, positivelycharged H⁺ ions need to diffuse across the p-n junction and through then-GaN layer that is exposed. However, because of the depletion field inthe p-n junction (with a direction from the n-type layer to the p-typelayer), positively charged H⁺ ions may not be able to diffuse from thep-type layer to the n-type layer across the p-n junction. Furthermore,hydrogen may have a much higher diffusion barrier and thus a much lowerdiffusivity in n-type GaN compared with in p-type GaN. Thus, thehydrogen ions may not diffuse through the n-type layer to the exposedtop surface of the n-type layer. Moreover, the activation may not beperformed right after the p-doping and before the growth of the activeregion either, because the subsequent growth may be performed in thepresence of high pressure ammonia (NH₃) in order to avoid decompositionof GaN at the high growth temperatures, and thus a semiconductor layer(e.g., the p-type semiconductor layer) that was activated may bere-passivated due to the presence of ammonia.

Therefore, in general, during the growth of the epitaxial layers, n-typesemiconductor layer 1114 may be grown first. P-type semiconductor layer1118 may be grown after the growth of active region 1116 to avoidcontamination of active region 1116 and facilitate activation of thedopants in the p-type semiconductor layer.

FIG. 11B shows a reflector layer 1120 and a bonding layer 1122 formed onp-type semiconductor layer 1118. Reflector layer 1120 may include, forexample, a metal layer such as an aluminum layer, a silver layer, or ametal alloy layer. In some embodiments, reflector layer 1120 may includea distributed Bragg reflector formed by conductive materials (e.g.,semiconductor materials or conductive oxides) or including conductivevias. In some embodiments, reflector layer 1120 may include one or moresublayers. Reflector layer 1120 may be formed on p-type semiconductorlayer 1118 in a deposition process. Bonding layer 1122 may include ametal layer, such as a titanium layer, a copper layer, an aluminumlayer, a gold layer, or a metal alloy layer. In some embodiments,bonding layer 1122 may include a eutectic alloy, such as Au—In, Au—Sn,Au—Ge, or Ag—In. Bonding layer 1122 may be formed on reflector layer1120 by a deposition process and may include one or more sublayers.

FIG. 11C shows a backplane wafer 1104 that includes a substrate 1130with electrical circuits formed thereon. The electrical circuits mayinclude digital and analog pixel drive circuits for driving individualmicro-LEDs. A plurality of metal pads 1134 (e.g., copper or tungstenpads) may be formed in a dielectric layer 1132 (e.g., including SiO₂ orSiN). In some embodiments, each metal pad 1134 may be an electrode(e.g., anode or cathode) for a micro-LED. In some embodiments, pixeldrive circuits for each micro-LED may be formed in an area matching thesize of a micro-LED (e.g., about 2 μm×2 μm), where the pixel drivecircuits and the micro-LED may collectively form a pixel of a micro-LEDdisplay panel. Even though FIG. 11C only shows metal pads 1134 formed inone metal layer in one dielectric layer 1132, backplane wafer 1104 mayinclude two or more metal layers formed in dielectric materials andinterconnected by, for example, metal vias, as in many CMOS integratedcircuits. In some embodiments, a planarization process, such as a CMPprocess, may be performed to planarize the exposed surfaces of metalpads 1134 and dielectric layer 1132. A bonding layer 1140 may be formedon dielectric layer 1132 and may be in physical and electrical contactwith metal pads 1134. As bonding layer 1122, bonding layer 1140 mayinclude a metal layer, such as a titanium layer, a copper layer, analuminum layer, a gold layer, a metal alloy layer, or a combinationthereof. In some embodiments, bonding layer 1140 may include a eutecticalloy. In some embodiments, only one of bonding layer 1140 or bondinglayer 1122 may be used.

FIG. 11D shows that micro-LED wafer 1102 and backplane wafer 1104 may bebonded together to form a wafer stack 1106. Micro-LED wafer 1102 andbackplane wafer 1104 may be bonded by the metal-to-metal bonding ofbonding layer 1122 and bonding layer 1140. The metal-to-metal bondingmay be based on chemical bonds between the metal atoms at the surfacesof the metal bonding layers. The metal-to-metal bonding may include, forexample, thermo-compression bonding, eutectic bonding, or transientliquid phase (TLP) bonding. The metal-to-metal bonding process mayinclude, for example, surface planarization, wafer cleaning (e.g., usingplasma or solvents) at room temperatures, and compression and annealingat elevated temperatures, such as about 250° C. or higher, to causediffusion of atoms. In eutectic bonding, a eutectic alloy including twoor more metals and with a eutectic point lower than the melting point ofthe two or more metals may be used for low-temperature wafer bonding.Because the eutectic alloy may become a liquid at the elevatedtemperature, eutectic bonding may be less sensitive to surface flatnessirregularities, scratches, particles contamination, and the like. Afterthe bonding, buffer layer 1112 and substrate 1110 may be thinned orremoved by, for example, etching, back grinding, or laser lifting, toexpose n-type semiconductor layer 1114.

FIG. 11E shows that wafer stack 1106 may be etched from the side of theexposed n-type semiconductor layer 1114 to form mesa structures 1108 forindividual micro-LEDs. As shown in FIG. 11E, the etching may includeetching through n-type semiconductor layer 1114, active region 1116,p-type semiconductor layer 1118, reflector layer 1120, and bondinglayers 1122 and 1140, in order to singulate and electrically isolatemesa structures 1108. Thus, each singulated mesa structure 1108 mayinclude n-type semiconductor layer 1114, active region 1116, p-typesemiconductor layer 1118, reflector layer 1120, and bonding layers 1122and 1140. To perform the etching, an etch mask layer may be formed onn-type semiconductor layer 1114. The etch mask layer may be patterned byaligning a photomask with the backplane wafer (e.g., using alignmentmarks on backplane wafer 1104) such that the patterned etch mask formedin the etch mask layer may align with metal pads 1134. Therefore,regions of the epitaxial layers and bonding layers above metal pads 1134may not be etched. Dielectric layer 1132 may be used as the etch-stoplayer for the etching. Even though FIG. 11E shows that mesa structures1108 have substantially vertical sidewalls, mesa structures 1108 mayhave other shapes as described above, such as a conical shape, aparabolic shape, or a truncated pyramid shape.

FIG. 11F shows that a passivation layer 1150 may be formed on sidewallsof mesa structures 1108, and a sidewall reflector layer 1152 may beformed on passivation layer 1150. Passivation layer 1150 may include adielectric layer (e.g., SiO₂, SiN, or Al₂O₃) or an undoped semiconductorlayer. Sidewall reflector layer 1152 may include, for example, a metal(e.g., Al) or a metal alloy. In some embodiments, gaps between mesastructures 1108 may be filled with a dielectric material 1154 and/or ametal. Passivation layer 1150, sidewall reflector layer 1152, and/ordielectric material 1154 may be formed using suitable depositiontechniques, such as chemical vapor deposition (CVD), physical vapordeposition (PVD), plasma-enhanced chemical vapor deposition (PECVD),atomic-layer deposition (ALD), laser metal deposition (LIVID), orsputtering. In some embodiments, sidewall reflector layer 1152 may fillthe gaps between mesa structures 1108. In some embodiments, aplanarization process may be performed after the deposition ofpassivation layer 1150, sidewall reflector layer 1152, and/or dielectricmaterial 1154. A common electrode layer 1160, such as a transparentconductive oxide (TCO) layer (e.g., an ITO layer) or a thin metal layerthat may be transparent to light emitted in active region 1116, may beformed on the n-type semiconductor layer 1114 to form n-contacts and acommon-cathode for the micro-LEDs.

FIGS. 12A-12E illustrate an example of a process of fabricating amicro-LED device according to certain embodiments. FIG. 12A shows amicro-LED wafer 1200 including epitaxial layers grown on a substrate1210. As described above, substrate 1210 may include, for example, aGaN, GaAs, or GaP substrate, or a substrate including, but not limitedto, sapphire, silicon carbide, silicon, zinc oxide, boron nitride,lithium aluminate, lithium niobate, germanium, aluminum nitride, lithiumgallate, partially substituted spinels, or quaternary tetragonal oxidessharing the beta-LiAlO₂ structure, where the substrate may be cut in aspecific direction to expose a specific plane (e.g., a c-plane or asemipolar plane) as the growth surface. In some embodiments, a bufferlayer may be formed on substrate 1210 to improve the lattice matchingbetween the growth substrate and the epitaxial layers, thereby reducingstress and defects in the epitaxial layers. The epitaxial layers mayinclude an n-type semiconductor layer 1220 (e.g., an n-doped GaN, AlInP,or AlGaInP layer), an active region 1230, and a p-type semiconductorlayer 1240 (e.g., a p-doped GaN, AlInP, or AlGaInP layer). Active region1230 may include multiple quantum wells or an MQW formed by thin quantumwell layers (e.g., InGaN layers or GaInP layers) sandwiched by barrierlayers (e.g., GaN layers, AlInP layers, or AlGaInP layers) as describedabove. The epitaxial layers may be grown layer-by-layer on substrate1210 or the buffer layer using techniques such as VPE, LPE, MBE, orMOCVD. In some embodiments, n-type semiconductor layer 1220 may be muchthicker than p-type semiconductor layer 1240.

FIG. 12B shows that micro-LED wafer 1200 may be etched from the side ofp-type semiconductor layer 1240 to form semiconductor mesa structures1202 for individual micro-LEDs. As shown in FIG. 12B, the etching mayinclude etching through p-type semiconductor layer 1240, active region1230, and at least a portion of n-type semiconductor layer 1220. Thus,each semiconductor mesa structure 1202 may include p-type semiconductorlayer 1240, active region 1230, and a portion of n-type semiconductorlayer 1220. To perform the etching, an etch mask layer may be formed onp-type semiconductor layer 1240, and dry or wet etching may be performedfrom the side of p-type semiconductor layer 1240. Due to the etchingfrom p-type semiconductor layer 1240, semiconductor mesa structure 1202may have sidewalls that are inwardly tilted in the z direction. Forexample, the angle between the sidewalls and the surface-normaldirection (the z direction) of micro-LED wafer 1200 may be between about0° to about 30°, such as about 15°. In some embodiments, semiconductormesa structures 1202 may have a conical shape, a parabolic shape, atruncated pyramid shape, or another shape. In some embodiments, afterthe etching, sidewalls of the etched semiconductor mesa structures 1202may be treated, for example, using KOH or an acid, to remove regionsthat may be damaged by high-energy ions during the dry etching.

FIG. 12C shows that micro-LED wafer 1200 may be further processed fromthe side of p-type semiconductor layer 1240 to form a wafer 1204 thatincludes an array of micro-LEDs. In the illustrated example, asemiconductor layer 1245 that may include the same base material (e.g.,a III-P material) as p-type semiconductor layer 1240, active region1230, and n-type semiconductor layer 1220 may be formed on sidewalls ofsemiconductor mesa structures 1202. Semiconductor layer 1245 may beundoped and may be formed on sidewalls of semiconductor mesa structures1202 through, for example, a regrow process that epitaxially growssemiconductor layer 1245 on sidewalls of semiconductor mesa structures1202. Semiconductor layer 1245 may be grown only on sidewalls ofsemiconductor mesa structures 1202 by using a mask layer on top ofsemiconductor mesa structures 1202 and in regions between semiconductormesa structures 1202. The undoped semiconductor layer 1245 may functionas a passivation layer that electrically isolate adjacent semiconductormesa structures 1202. The high-refractive index semiconductor layer 1245that has the same base material as active region 1230 (e.g., with arefractive index greater than about 3.0, such as 3.4, for red light) mayincrease the optical distance from active region 1230 to a sidewallmetal reflector. Epitaxially growing semiconductor layer 1245 that hasthe same base material as active region 1230 on sidewalls of activeregion 1230 may also reduce the defects at the etched sidewalls ofactive region 1230, and thus may reduce non-radiative recombination atthe sidewall regions of active region 1230 and improve the internalquantum efficiency of the micro-LEDs. In some embodiments, additionallayers (e.g., including dielectric, doped semiconductor material,undoped semiconductor material, or a combination) may be formed onsemiconductor layer 1245. The additional layers may have refractiveindices lower than the refractive index of semiconductor layer 1245.

After the growth of semiconductor layer 1245, a passivation layer 1250may be formed on surfaces of micro-LED wafer 1200, including surfaces ofsemiconductor layer 1245, to further electrically isolate semiconductormesa structures 1202. Passivation layer 1250 may include a dielectricmaterial, such as SiO₂, Al₂O₃, or Si₃N₄. In some embodiments,passivation layer 1250 may have a thickness greater than about 50 nm,such as about 100 nm or thicker. A reflective metal layer 1252 (e.g.,Al, Au, Ag, Cu, Ti, Ni, Pt, or a combination thereof) may be formed onpassivation layer 1250 to optically isolate individual micro-LEDs andimprove the light extraction efficiency. A dielectric material 1260(e.g., SiO₂) may be deposited on reflective metal layer 1252 and regionsbetween semiconductor mesa structures 1202. Passivation layer 1250,reflective metal layer 1252, and dielectric material 1260 may be formedusing suitable deposition techniques, such as chemical vapor deposition(CVD), physical vapor deposition (PVD), plasma-enhanced chemical vapordeposition (PECVD), atomic-layer deposition (ALD), laser metaldeposition (LIVID), or sputtering. A back reflector and p-contact 1262may be form in dielectric material 1260 and may contact p-typesemiconductor layer 1240 of a corresponding semiconductor mesa structure1202. Back reflector and p-contact 1262 may include, for example, Au,Ag, Al, Ti, Cu, Ni, ITO, or a combination thereof. Even though not shownin FIG. 12C, in some embodiments, one or more metal interconnect layersmay be formed on back reflector and p-contact 1262. The one or moremetal interconnect layers may include a bonding layer that includesmetal bonding pads in a dielectric layer as described above with respectto, for example, FIG. 9B.

FIG. 12D shows that wafer 1204 may be bonded to a backplane wafer 1206in a hybrid bonding process. Backplane wafer 1206 may include asubstrate 1270 with electrical circuits formed thereon. The electricalcircuits may include digital and analog pixel drive circuits for drivingindividual micro-LEDs. A plurality of metal pads 1272 (e.g., copper ortungsten pads) may be formed in a dielectric layer 1274 (e.g., includingSiO₂ or SiN). In some embodiments, each metal pad 1272 may be anelectrode (e.g., anode or cathode) for a micro-LED. Even though FIG. 12Donly shows metal pads 1272 formed in one metal layer in one dielectriclayer 1274, backplane wafer 1206 may include two or more metal layersformed in dielectric materials and interconnected by, for example, metalvias, as in many CMOS integrated circuits.

As described above with respect to, for example, FIGS. 8A-8D, thebonding surfaces of wafer 1204 and backplane wafer 1206 may beplanarized, cleaned, and activated before the bonding. Wafer 1204 may beturned upside down and brought into contact with backplane wafer 1206such that dielectric layer 1274 and dielectric material 1260 may be indirect contact and may be bonded together with or without heat treatmentdue to the surface activation. In some embodiments, a compressionpressure may be applied to wafer 1204 and backplane wafer 1206 such thatthe bonding layers are pressed against each other. After the bonding ofthe dielectric materials, an annealing process may be performed at anelevated temperature to bond the metal pads (e.g., back reflector andp-contacts 1262 and metal pads 1272) at the bonding surfaces.

FIG. 12E shows that, after the bonding of wafer 1204 and backplane wafer1206, substrate 1210 of wafer 1204 may be removed, and a transparentconductive oxide (TCO) layer 1280 (e.g., such as an ITO layer) may beformed on the exposed n-type semiconductor layer 1220. TCO layer 1280may form a common cathode for the micro-LEDs. In the illustratedexample, non-native lenses 1290 may be fabricated in a dielectricmaterial (e.g., SiN or SiO₂) or an organic material, and may be bondedto TCO layer 1280. In some embodiments, non-native lenses 1290 may befabricated in a dielectric material deposited on TCO layer 1280. In someembodiments, native lenses may be fabricated in n-type semiconductorlayer 1220, and the common cathode may be formed on the native lensesand/or may be the portion of n-type semiconductor layer 1220 that hasnot been etched (which may be heavily doped to reduce the resistance).As shown in FIGS. 12D and 12E, since wafer 1204 is turned upside downand bonded to backplane wafer 1206 and light may exit the micro-LEDsfrom the side of n-type semiconductor layer 1220, the semiconductor mesastructures of the micro-LEDs may have sidewalls that are outwardlytilted in the light emitting direction (e.g., the z direction).

As described above, the internal quantum efficiency of an LED may dependon the relative rates of competitive radiative (light producing)recombination and non-radiative (lossy) recombination that occur in theactive region of the LED. Non-radiative recombination processes in theactive region include Shockley-Read-Hall (SRH) recombination at defectsites, and electron-electron-hole (eeh) and/or electron-hole-hole (ehh)Auger recombination. The Auger recombination is a non-radiative processinvolving three carriers, which affects all sizes of LEDs. Inmicro-LEDs, because the lateral size of each micro-LED may be comparableto the minority carrier diffusion length, a larger proportion of thetotal active region may be within a minority carrier diffusion lengthfrom the LED sidewall surfaces where the defect density and thedefect-induced non-radiative recombination rate may be high. Therefore,a larger proportion of the injected carriers may diffuse to the regionsnear the sidewall surfaces, where the carriers may be subjected to ahigher SRH recombination rate. This may cause the efficiency of the LEDto decrease (in particular, at low current injection), cause the peakefficiency of the LED to decrease, and/or cause the peak efficiencyoperating current to increase. Increasing the injected current may causethe efficiencies of the micro-LEDs to drop due to the higher eeh or ehhAuger recombination rate at a higher current density, and may also causespectral shift of the emitted light. As the physical sizes of LEDs arefurther reduced, efficiency losses due to surface recombination near theetched sidewall facets that include surface imperfections may becomemuch more significant. III-phosphide materials, such as AlGaInP, canhave a high surface recombination velocity and minority carrierdiffusion length. For example, carriers in AlGaInP can have highdiffusivity (mobility), and AlGaInP may have an order of magnitudehigher surface recombination velocity than III-nitride materials. Thus,the internal and external quantum efficiencies of AlGaInP-based redlight-emitting LEDs may drop even more significantly as the device sizereduces.

At the light-emitting surface of an LED, such as the interface betweenthe LED and air, some incident light with incident angles smaller thanthe critical angle for total internal reflection (TIR) may be refractedto exit the LED, but incident light with incident angles greater thanthe critical angle may be reflected back to the LED due to totalinternal reflection. Light incident on the back reflector and mesasidewall reflectors may be specularly reflected. Some light may bereflected by the back reflector and mesa sidewall reflectors towards thelight-emitting surface, but some light may be trapped in the LED.Because of the specular reflection and the geometry of the LED mesastructure, there may be no light mixing within the LED, which may resultin closed orbits for light within the LED. Light trapped in themicro-LED may eventually be absorbed by the LED. For example, sometrapped light may be absorbed by the semiconductor materials to generateelectron-hole pairs, which may recombine radiatively or non-radiatively.Some trapped light may be absorbed by metals (e.g., metal contacts orreflectors) at the bottom and/or sidewalls of the LED due to, forexample, surface plasmon resonance that may be excited by p-polarizedlight at the interface between a metal layer and a dielectric layer(e.g., the passivation layer) or a semiconductor material layer.Therefore, techniques for improving the light extraction efficiency ofthe LED may be desired.

In large LEDs, the light extraction efficiency may be improved using,for example, thin film technology, or patterned sapphire substrates withdense, periodic patterns on the substrate surfaces. For example,patterned sapphire substrate techniques may cause light randomization inthe semiconductor layer, such that the propagation directions of thephotons that may otherwise be trapped in the mesa structure may berandomized to increase the possibility of being released from theconfinement and exiting the mesa structure. Therefore, the overall lightextraction efficiency may be improved. However, these techniques may notbe used in micro-LEDs with linear dimensions less than, for example,about 20 μm or about 10 μm, due to the small sizes and high aspectratios (height vs width) of these micro-LEDs.

Micro-lenses may be used to extract and collimate light emitted fromLEDs to increase the total LEEs (e.g., for extracted light with emissionangles within ±90°) and the collected LEEs (e.g., for extracted lightwith emission angles within ±18.5°) of LEDs in a near-eye display.Non-native lenses made from, for example, SiN, SiO₂, or organicmaterials, may be easier to fabricate than native lenses fabricated in athick semiconductor layer of an LED, but may exhibit lower collectedLEEs compared with native lenses due to, for example, the refractiveindex mismatch between the non-native lens and the LED, which may causeFresnel reflection (and total internal reflection) at the interfacebetween the LED and the non-native lens.

FIG. 13 illustrates an example of light refraction at an interfacebetween a micro-LED 1310 and another medium 1320 that may have a lowerrefractive index than the semiconductor material of micro-LED 1310. Thesemiconductor material of micro-LED 1310 may have a refractive index n₁,such as about 2.4 for GaN-based semiconductor materials or about 3.5 forAlGaInP-based semiconductor materials. Medium 1320 may have a refractiveindex n₂, such as about 1 for air, about 1.5 for SiO₂, or about 2.0 forSiN. A surface 1330 shown in FIG. 13 may be the light emitting surfaceof micro-LED 1310 or the interface between a micro-LED and a non-nativelens made of, for example, undoped silica glass (USG) or siliconnitride.

The critical angle θ_(c) for total internal reflection of light incidenton surface 1330 from micro-LED 1310 may be

$\theta_{c} = {{\sin}^{- 1}{\left( \frac{n_{2}}{n_{1}} \right).}}$Only incident light that is within the critical angle θ_(c), which maybe a small portion of the light emitted by the active region ofmicro-LED 1310, may be refracted into medium 1320. The lighttransmittance at surface 1330 may be determined by:

${{P_{T}/P_{0}} = {\frac{1}{1 - {\cos\theta_{c}}}{\int}_{0}^{\pi/2}\sin\theta \times {T(\theta)}d\theta}},$where P₀ is the total power of the incident light, P_(T) is be the totalpower of the transmitted light, and T(θ) is the transmittance forincident light with an incident angle θ In addition, only light emittedinto medium 1320 with emission angles within the acceptance cone (e.g.,about)±18.5° of the display optics of a display system may be collectedby the display optics, where the total power of the collected lightP_(c) may a small portion of the total power of the transmitted lightP_(T).

In III-phosphide-based LEDs, such as some red light-emittingIII-phosphide LEDs, the refractive indices of the III-phosphidesemiconductor materials (e.g., GaP, InP, GaInP, or AlGaInP) may begreater than about 3.0 (e.g., about 3.4 or 3.5) for visible light, muchhigher than the refractive indices of many III-nitride semiconductormaterials (e.g., about 2.4 for GaN). Therefore, the critical angle fortotal internal reflection at the interface between the III-phosphidesemiconductor material and an adjacent lower refractive index material(e.g., air or a dielectric) may be much smaller than the critical anglefor total internal reflection at the interface between a III-nitridesemiconductor material and the lower refractive index material. Forexample, the critical angle may be about 25° at the interface between aGaN-based micro-LED and air, about 16° at the interface between anAlGaInP-based micro-LED and air, about 39° at the interface between aGaN-based micro-LED and SiO₂, about 25° at the interface between anAlGaInP-based micro-LED and SiO₂, about 56° at the interface between aGaN-based micro-LED and SiN, or about 35° at the interface between anAlGaInP-based micro-LED and SiN. As such, more light emitted in theactive region of a III-phosphide-based LED may be trapped in the LED andmay be absorbed eventually. Therefore, the LEE of a red light-emittingIII-phosphide LED may be low.

FIG. 14A illustrates an example of a micro-LED device 1400 including amicro-LED and a micro-lens fabricated in a low-refractive index materiallayer. Micro-LED device 1400 may be configured to emit red light. In theillustrated example, micro-LED device 1400 may include a semiconductormesa structure that includes a portion of an n-type semiconductor layer1430 (e.g., including n-dope AlGaInP), an active region 1420 (e.g.,including one or more QWs formed by GaInP quantum well layers andAlGaInP quantum barrier layers), and a p-type semiconductor layer 1410(e.g., including p-dope AlGaInP). A passivation layer 1440 may be onsidewalls of the semiconductor mesa structure. Passivation layer 1440may include a dielectric material that may have a refractive index muchlower than the refractive indices of n-type semiconductor layer 1430,active region 1420, and p-type semiconductor layer 1410. In theillustrated example, passivation layer 1440 may include SiN. A backreflector and p-contact 1450 may be at the bottom of the semiconductormesa structure and may be coupled to p-type semiconductor layer 1410. Areflective metal layer (e.g., including Al) may be formed on passivationlayer 1440. Regions surrounding the semiconductor mesa structure may befilled with a metal material (e.g., Al or Cu) and/or a dielectricmaterial (e.g., SiO₂). A non-native micro-lens 1462 may be formed in asubstrate 1460 and may be bonded to n-type semiconductor layer 1430. Insome embodiments, non-native micro-lens 1462 may include anantireflective coating (not shown in FIG. 14A). Active region 1420 ofthe micro-LED in micro-LED device 1400 may have a width less than about1.2 μm or less than about 1 μm.

FIG. 14B includes a graph 1470 illustrating light transmittance T(θ) asa function of the angle of incidence θ at an interface between anAlInGaP-based micro-LED and a SiN micro-lens as shown in FIG. 14A. TheAlInGaP-based micro-LED may be configured to emit light with a centralwavelength about 625 nm (red light). As described above, the criticalangle θ_(c) at the interface between the AlInGaP-based micro-LED and theSiN micro-lens may be about 35° as indicated by a line 1474. A curve1472 in FIG. 14B shows that the light transmittance T(θ) may be high(e.g., >80%) for incident light with small incident angles (e.g., <30°),but may be zero for incident light with incident angles greater than thecritical angle.

FIG. 14C includes a graph 1480 illustrating light transmittance as afunction of the angle of emission at an interface between anAlInGaP-based micro-LED and a SiN micro-lens as shown in FIG. 14A. Acurve 1482 in FIG. 14C shows that the light transmittance may be high(e.g., >80%) for emitted light with emission angles smaller than about70° (which corresponds to incident angles less than about 32°). Thus, alarge portion of the emitted light may have emission angles greater thanabout 18.5° as indicated by a line 1484, and thus may not be collectedby the display optics if the emitted light is not properly collimated.

In the example shown in FIG. 13 , when no micro-lenses are used and themicro-LED includes an AlInGaP-based micro-LED configured to emit redlight, the light extraction efficiency may be about 4% for all extractedlight (with emission angles within ±90°), and the collected lightextraction efficiency may be about 0.4% for extracted light withemission angles within ±18.5°. When non-native micro-lens 1462 (e.g., aSiN micro-lens) is used and the micro-LED includes an AlInGaP-basedmicro-LED configured to emit red light, the light extraction efficiencymay be about 5.8% for all extracted light (with emission angles within±90°), and the collected light extraction efficiency may be about 0.9%for extracted light with emission angles within ±18.5°. Thus, even if amicro-lens is used to extract and collimate light emitted by amicro-LED, the total LEE and the collected LEE may still be very low forsmall micro-LEDs.

According to certain embodiments, to improve the light extractionefficiency, a micro-LED device may include a micro-lens bonded to asemiconductor layer of a micro-LED through a bonding layer, where themicro-lens may have a refractive index close to or greater than therefractive index of the semiconductor layer, and thus may preserve orreduce the emission angles of the light emitted by the micro-LED. Thebonding layer may include a dielectric material such as an oxide (e.g.,SiO₂) or a nitride (e.g., SiN), or a transparent conductive oxide (e.g.,ITO). A maximum optical thickness of the bonding layer may be, forexample, an integer multiple of a half-wavelength (λ/2) of the lightemitted by the micro-LED, and the bonding layer may function as anoptical thin film filter that may filter (e.g., reflect) light withlarge incident angles, such that the light entering the micro-lens maybe incident light with small incident angles. In some embodiments, thebonding layer may be thin (e.g., λ/10 or λ/20) such that the totalinternal reflection at the interface between the bonding layer and thesemiconductor layer of the micro-LED may be reduced or frustrated. Assuch, the total LEE for extracted light with emission angles within ±90°may be increased. The micro-lens may collimate the emitted light suchthat the emitted light may have small emission angles and may be moreefficiently collected by the display optics. Since the refractive indexof the micro-lens may be close to or greater than the refractive indexof the semiconductor layer, the light entering and propagating in themicro-lens may have propagation angles similar to or smaller than theincident angles in the micro-LED.

FIG. 15A illustrates an example of a micro-LED device 1500 including anAlInGaP-based micro-LED and a micro-lens 1570 fabricated in an AlInGaPlayer and bonded to the AlInGaP-based micro-LED through a dielectricbonding layer 1560. The AlInGaP-based micro-LED may be fabricated usingthe techniques described above with respect to, for example, FIGS.11A-11F. In the example illustrated in FIG. 15A, the AlInGaP-basedmicro-LED may include a semiconductor mesa structure that includes ann-type semiconductor layer 1530 (e.g., including n-dope AlGaInP), anactive region 1520 (e.g., including one or more QWs formed by GaInPquantum well layers and AlGaInP quantum barrier layers), and a p-typesemiconductor layer 1510 (e.g., including p-dope AlGaInP). A passivationlayer 1540 may be formed on sidewalls of the semiconductor mesastructure. Passivation layer 1540 may include a dielectric material thatmay have a refractive index much lower than the refractive indices ofn-type semiconductor layer 1530, active region 1520, and p-typesemiconductor layer 1510, and thus total internal reflection may occurat the interface between passivation layer 1540 and the semiconductormesa structure. Passivation layer 1540 may include, for example, SiN,SiO₂, or Al₂O₃. A back reflector and p-contact 1550 may be at the bottomof the semiconductor mesa structure and may be coupled to p-typesemiconductor layer 1510. In some embodiments, a reflective metal layer(e.g., including Al) may be formed on passivation layer 1540. Regionssurrounding the semiconductor mesa structure may be filled with a metalmaterial (e.g., Al or Cu) and/or a dielectric material (e.g., SiO₂).

Micro-lens 1570 may be fabricated in an AlGaInP substrate (or anothermaterial layer having a high refractive index), and may be bonded ton-type semiconductor layer 1530 through dielectric bonding layer 1560.In some embodiments, micro-lens 1570 may include an antireflectivecoating (not shown in FIG. 15A). Micro-lens 1570 may have a lineardimension greater than a linear dimension of active region 1520. Forexample, the linear dimension of micro-lens 1570 may be close to thepitch of a micro-LED array. Dielectric bonding layer 1560 may include,for example, SiN or SiO₂. In some embodiments, a TCO (e.g., ITO) may beused in the bonding layer. Dielectric bonding layer 1560 may have anoptical thickness of about a half-wavelength of the emitted light, oranother integer multiple of the half-wavelength of the emitted light.Dielectric bonding layer 1560 may function as an optical thin filmfilter that may reflect light with large incident angles as shown inFIG. 15A, such that the light entering micro-lens 1570 may be incidentlight with small incident angles in the micro-LED. Since the refractiveindex of micro-lens 1570 may be close to or greater than the refractiveindex of n-type semiconductor layer 1530, the light entering andpropagating in micro-lens 1570 may have propagation angles (with respectto the z direction) similar to or less than the incident angles inn-type semiconductor layer 1530 as shown in FIG. 15A. Thus, lightpropagating in micro-lens 1570 may have small propagation angles and maybe better collimated by micro-lens 1570, such that light transmitted outof micro-lens 1570 may have small emission angles and may be moreefficiently collected by the display optics.

FIG. 15B illustrates another example of a micro-LED device 1502including an AlInGaP-based micro-LED, and a micro-lens 1572 fabricatedin an AlInGaP layer and bonded to the AlInGaP-based micro-LED through abonding layer 1562. The AlInGaP-based micro-LED may be fabricated usingthe techniques described above with respect to, for example, FIGS.12A-12E. In the example illustrated in FIG. 15B, the AlInGaP-basedmicro-LED may include a semiconductor mesa structure that includes aportion of an n-type semiconductor layer 1532 (e.g., including n-dopeAlGaInP), an active region 1522 (e.g., including one or more QWs formedby GaInP quantum well layers and AlGaInP quantum barrier layers), and ap-type semiconductor layer 1512 (e.g., including p-dope AlGaInP). Apassivation layer 1542 may be formed on sidewalls of the semiconductormesa structure. Passivation layer 1542 may include a dielectric materialthat may have a refractive index much lower than the refractive indicesof the semiconductor materials of the semiconductor mesa structure, andthus total internal reflection may occur at the interface betweenpassivation layer 1542 and the semiconductor mesa structure. Passivationlayer 1542 may include, for example, SiN, SiO₂, or Al₂O₃. A backreflector and p-contact 1552 may be at the bottom of the semiconductormesa structure and may be coupled to p-type semiconductor layer 1512. Insome embodiments, a reflective metal layer (e.g., including Al) may beformed on passivation layer 1542. Regions surrounding the semiconductormesa structure may be filled with a metal material (e.g., Al or Cu)and/or a dielectric material (e.g., SiO₂).

Micro-lens 1572 may be fabricated in an AlGaInP substrate (or anothermaterial layer having a high refractive index), and may be bonded ton-type semiconductor layer 1532 through bonding layer 1562. In someembodiments, micro-lens 1572 may include an antireflective coating (notshown in FIG. 15B). Micro-lens 1572 may have a linear dimension greaterthan a linear dimension of active region 1522. For example, the lineardimension of micro-lens 1572 may be close to the pitch of a micro-LEDarray. Bonding layer 1562 may include SiN, SiO₂, or a TCO (e.g., ITO),and may have an optical thickness of about a half-wavelength of theemitted light, or another integer multiple of the half-wavelength of theemitted light. Bonding layer 1562 may function as an optical thin filmfilter that may reflect light with large incident angles as shown inFIG. 15B, such that the light entering micro-lens 1572 may be incidentlight with small incident angles in the micro-LED. Since the refractiveindex of micro-lens 1572 may be close to or greater than the refractiveindex of n-type semiconductor layer 1532, the light entering andpropagating in micro-lens 1572 may have propagation angles (with respectto the z direction) similar to or less than the incident angles inn-type semiconductor layer 1532. Thus, light propagating in micro-lens1572 may have small propagation angles and may be better collimated bymicro-lens 1572, such that light transmitted out of micro-lens 1572 mayhave small emission angles and may be more efficiently collected by thedisplay optics.

FIGS. 16A-16C illustrate light transmittance from an AlInGaP-basedmicro-LED to an AlInGaP micro-lens, through SiN bonding layers ofdifferent thicknesses, as a function of the angle of incidence in amicro-LED device as shown in FIG. 15B. Because the micro-LED and themicro-lens (e.g., micro-lens 1572) may have the same refractive index,the angle of incidence in the micro-LED and the angle of emission in themicro-lens may be the same. The SiN bonding layer (e.g., bonding layer1562) may filter the incident light such that only light with smallincident angles (e.g., less than about 35°) may pass through the SiNbonding layer and enter the micro-lens. In the example shown in FIG.16A, the optical thickness of the SiN bonding layer may be about onewavelength (λ) of the emitted light (e.g., red light with a centerwavelength λ=625 nm), and the transmitted light with emission angleswithin ±18.5° may be about 31% of the total transmitted light. In theexample shown in FIG. 16B, the optical thickness of the SiN bondinglayer may be about one half-wavelength (212) of the emitted light (e.g.,red light with a center wavelength λ=625 nm), and the transmitted lightwith emission angles within ±18.5° may be about 30% of the totaltransmitted light. In the example shown in FIG. 16C, the opticalthickness of the SiN bonding layer may be about three half-wavelengths(3λ/2) of the emitted light (e.g., λ=625 nm), and the transmitted lightwith emission angles within ±18.5° may be about 31.2% of the totaltransmitted light.

FIGS. 17A and 17B illustrate light transmittance from an AlInGaP-basedmicro-LED to an AlInGaP micro-lens, through SiO₂ bonding layers ofdifferent thicknesses, as a function of the angle of incidence in amicro-LED device as shown in FIG. 15B. Because the micro-LED and themicro-lens (e.g., micro-lens 1572) may have the same refractive index,the angle of incidence in the micro-LED and the angle of emission in themicro-lens may be the same. The SiO₂ bonding layer (e.g., bonding layer1562) may filter the incident light such that only light with smallincident angles (e.g., less than about 25°) may pass through the SiNbonding layer and enter the micro-lens. In the example shown in FIG.17A, the optical thickness of the SiO₂ bonding layer may be about onewavelength (λ) of the emitted light (e.g., red light with λ=625 nm), andthe transmitted light with emission angles within ±18.5° may be about58% of the total transmitted light. In the example shown in FIG. 17B,the optical thickness of the SiO₂ bonding layer may be about onehalf-wavelength (λ/2) of the emitted light (e.g., red light with λ=625nm), and the transmitted light with emission angles within ±18.5° may beabout 60% of the total transmitted light.

FIG. 18A illustrates an example of a micro-LED device 1800 including anAlInGaP-based micro-LED and a micro-lens 1850 fabricated in an AlInGaPlayer and bonded to the AlInGaP-based micro-LED through a SiO₂ bondinglayer. The AlInGaP-based micro-LED may be fabricated using thetechniques described above with respect to, for example, FIGS. 12A-12E.In the example illustrated in FIG. 18A, the AlInGaP-based micro-LED mayinclude semiconductor layers 1810, which may include an n-typesemiconductor layer (e.g., including n-dope AlGaInP), an active region(e.g., including one or more QWs formed by GaInP quantum well layers andAlGaInP quantum barrier layers), and a p-type semiconductor layer (e.g.,including p-dope AlGaInP). Semiconductor layers 1810 may be epitaxiallygrown on growth substrate, and may be etched to form semiconductor mesastructures for individual micro-LEDs. A passivation layer 1820 may bedeposited on sidewalls of the semiconductor mesa structure. Passivationlayer 1820 may include a dielectric material that may have a refractiveindex lower than the refractive indices of semiconductor layers 1810,and thus total internal reflection may occur at the interface betweenpassivation layer 1820 and the semiconductor mesa structure. Passivationlayer 1820 may include, for example, SiN, SiO₂, or Al₂O₃. A backreflector and contact 1830 may be at the bottom of the semiconductormesa structure and may be coupled to a semiconductor layer ofsemiconductor layers 1810 (e.g., p-type or n-type semiconductor layer).In some embodiments, back reflector and contact 1830 may include analuminum layer and an ITO layer. In some embodiments, a reflective metallayer (e.g., including Al) may be formed on passivation layer 1820.Regions surrounding the semiconductor mesa structure may be filled witha metal material (e.g., Al or Cu) and/or a dielectric material (e.g.,SiO₂).

Micro-lens 1850 may be fabricated in an AlGaInP material layer, and maybe bonded to semiconductor layers 1810 through dielectric bonding layer1840. In some embodiments, micro-lens 1850 may include an antireflectivecoating (not shown in FIG. 18A). Micro-lens 1850 may have a width (e.g.,about 2 μm) greater than a width (e.g., about 1 μm) of the semiconductormesa structure. For example, the width of micro-lens 1850 may be closeto the pitch of a micro-LED array. Dielectric bonding layer 1840 mayinclude SiN or SiO₂, and may have an optical thickness of about ahalf-wavelength of the emitted light, or another integer multiple of thehalf-wavelength of the emitted light. Dielectric bonding layer 1840 mayfunction as an optical thin film filter that may reflect light withlarge incident angles, such that the light entering micro-lens 1850 maybe incident light with small incident angles in the micro-LED. In someembodiments, a TCO (e.g., ITO) layer may be used as the bonding layer.Since the refractive index of micro-lens 1850 may be close to or greaterthan the refractive index of semiconductor layers 1810, the lightentering and propagating in micro-lens 1850 may have propagation angles(with respect to the z direction) similar to or less than the incidentangles in semiconductor layers 1810. Thus, light propagating inmicro-lens 1850 may have small propagation angles and may be bettercollimated by micro-lens 1850, such that light transmitted out ofmicro-lens 1850 may have small emission angles and may be moreefficiently collected by the display optics.

FIG. 18B illustrates a beam profile of the light beam emitted by anexample of the micro-LED device of FIG. 18A. A curve 1860 in FIG. 18Bshows the power density of the extracted light as a function of theemission angle for emitted light of a certain wavelength (e.g., a centerwavelength about 625 nm). A curve 1870 shows the spectrally integratedpower density of the extracted light as a function of the emissionangle, where the full-width at half-maximum (FWHM) of the spectrum ofthe emitted light may be about 20 nm. The total light extractionefficiency for extracted light with emission angles within ±90° may beabout 18.1%, and the collected light extraction efficiency for extractedlight with emission angles within ±18.5° may be about 1.4%. Thiscollected LEE may be higher than the collected LEE of micro-LED device1400 (e.g., about 0.9%), but is still very low.

According to certain embodiments, to further improve the collected lightextraction efficiency, the semiconductor layer bonded to the micro-lensthrough the dielectric binding layer may have an uneven top surface,where the portion of the semiconductor layer at the center region ofeach micro-LED may have a higher thickness to form a mesa structure.Therefore, the bonding layer between the micro-LED and the micro-lensmay have a lower optical thickness (e.g., less than about λ/5, such asless than about λ/10, less than about λ/20, or less than about 20-30 nm)that may promote frustrated total internal reflection at the centerregion of the micro-LED device. As such, the total LEE for extractedlight with emission angles within ±90° may be increased due to, forexample, the frustrated TIR of the bonding layer and the refractiveindex matching between the micro-LED and the micro-lens.

In addition, the mesa structure of the semiconductor layer at the centerregion of the micro-LED may have a size smaller that the active regionof the LED. Therefore, light emitted in the active region of themicro-LED may be concentrated in the smaller-sized mesa structure of thesemiconductor layer at the center region of the micro-LED, due to alarge refractive index difference between the semiconductor layer andthe bonding layer. The concentrated light may be more effectivelycollimated by the micro-lens to have small emission angles because themesa structure of the semiconductor layer at the center region of themicro-LED may function as a point light source at a focal point of themicro-lens. As a result, a higher percentage of the extracted light mayhave emission angles within a small emission cone (e.g., within ±18.5°)and thus may be collected by the display optics. Therefore, thecollected light extraction efficiency (e.g., for extracted light withemission angles within ±18.5°) may also be improved.

FIG. 19A illustrates an example of a micro-LED device 1900 including anAlInGaP-based micro-LED and a micro-lens 1950 fabricated in an AlInGaPlayer and bonded to the AlInGaP-based micro-LED through a bonding layerthat has a low thickness at the center of the micro-LED device accordingto certain embodiments. Micro-LED device 1900 may be a pixel in an arrayof pixels formed by an array of micro-LEDs and an array of micro-lenses.The AlInGaP-based micro-LED may be fabricated using the techniquesdescribed above with respect to, for example, FIGS. 12A-12E. In theexample illustrated in FIG. 19A, the AlInGaP-based micro-LED may includesemiconductor layers 1910, which may include an n-type semiconductorlayer (e.g., including n-dope AlGaInP), an active region (e.g.,including one or more QWs formed by GaInP quantum well layers andAlGaInP quantum barrier layers), and a p-type semiconductor layer (e.g.,including p-dope AlGaInP) grown on a substrate to form a micro-LEDwafer. Semiconductor layers 1910 may be etched from one side (e.g., theside of the p-type semiconductor layer) to form semiconductor mesastructures for individual micro-LEDs on the micro-LED wafer. Apassivation layer 1920 may be deposited on sidewalls of thesemiconductor mesa structures. Passivation layer 1920 may include adielectric material that may have a refractive index lower than therefractive indices of semiconductor layers 1910, and thus total internalreflection may occur at the interface between passivation layer 1920 andthe semiconductor mesa structure. Passivation layer 1920 may include,for example, SiN, SiO₂, or Al₂O₃. A back reflector and contact 1930 maybe at the bottom of the semiconductor mesa structure and may be coupledto a semiconductor layer of semiconductor layers 1910 (e.g., p-type orn-type semiconductor layer). In some embodiments, back reflector andcontact 1930 may include an aluminum layer and an ITO layer. In someembodiments, a reflective metal layer (e.g., including Al) may be formedon passivation layer 1920. Regions surrounding the semiconductor mesastructure may be filled with a metal material (e.g., Al or Cu) and/or adielectric material (e.g., SiO₂).

After forming the semiconductor mesa structures, passivation layer 1920,and back reflector and contact 1930, the micro-LED wafer may be bondedto a backplane wafer as shown in FIG. 12D, and the substrate of themicro-LED wafer may be removed to expose a semiconductor layer ofsemiconductor layers 1910, such as the n-type semiconductor layer. Theexposed semiconductor layer (e.g., the n-type semiconductor layer) maybe etched to form second mesa structures 1912 at centers of themicro-LEDs. Second mesa structures 1912 may have, for example, acylindrical shape, a truncated cone shape, a truncated pyramid shape, orparabolic sidewalls. The maximum optical thickness (in the z direction)of second mesa structures 1912 may be close to, for example, an integermultiple of the half-wavelength (λ/2) of the emitted light. The width(in the x direction) of second mesa structures 1912 may be smaller thanthe width of the semiconductor mesa structure (e.g., the active regionin the semiconductor mesa structure). In one example, the width of thesemiconductor mesa structure may be about 1 μm, while the width ofsecond mesa structure 1912 may be less than about 0.8 μm, such as about0.6 μm.

Micro-lens 1950 may be formed in an AlGaInP material layer and may bebonded to semiconductor layers 1910, through a bonding layer 1940. Insome embodiments, micro-lens 1950 may include an antireflective coating(not shown in FIG. 19A). Micro-lens 1950 may have a width (e.g., about 2μm) greater than a width (e.g., about 1 μm) of the semiconductor mesastructure. For example, the width of micro-lens 1950 may be close to thepitch of a micro-LED array. In some embodiments, a ratio between thewidth of micro-lens 1950 and the width of second mesa structure 1912 maybe greater than about 2 or greater than about 3. Bonding layer 1940 mayinclude, for example, SiN, SiO₂, or a TCO (e.g., ITO). The maximumphysical thickness of bonding layer 1940 may be an integer multiple ofthe half-wavelength of the emitted light divided by the refractive indexof bonding layer 1940. Therefore, the thickness of bonding layer 1940 ontop of second mesa structures 1912 may be very thin, such as having anoptical thickness less than about ⅕, 1/10, or 1/20 of the wavelength λof the emitted light (e.g., less than about 50 nm, such as about 25 nm),and thus may frustrate total internal reflection such that incidentlight with incident angles greater than the critical angle may stillpass through bonding layer 1940 and enter micro-lens 1950. Since therefractive index of micro-lens 1950 may be close to or greater than therefractive index of semiconductor layers 1910, light entering andpropagating in micro-lens 1950 may have propagation angles (with respectto the z direction) similar to or less than the incident angles insemiconductor layers 1910. Since second mesa structure 1912 ofsemiconductor layer 1910 at the center region of the micro-LED may havea size much smaller that the active region of the micro-LED and lightemitted in the active region of the micro-LED may be concentrated insecond mesa structure 1912 of semiconductor layer 1910 due to a largerefractive index difference between semiconductor layers 1910 (e.g.,n=3.5) and bonding layer 1940 (e.g., n=1.5−2.0), second mesa structure1912 of semiconductor layer 1910 at the center region of the micro-LEDmay be close to a point light source for the micro-lens. Micro-lens 1950may be positioned (e.g., by controlling the thickness of the AlGaInPmaterial layer in which micro-lens 1950 is fabricated) such that secondmesa structure 1912 may be at the focal point of micro-lens 1950, andthus the light transmitted by the active region and concentrated insecond mesa structure 1912 may be better collimated by micro-lens 1950to have small emission angles.

FIG. 19B illustrates another example of a micro-LED device 1902including an AlInGaP-based micro-LED and micro-lens 1950 fabricated inan AlInGaP layer and bonded to the AlInGaP-based micro-LED throughbonding layer 1940 that has a low thickness at the center of themicro-LED device according to certain embodiments. Micro-LED device 1902may be similar to micro-LED device 1900, but may include second mesastructure 1914 with tilted sidewalls in semiconductor layers 1910.

FIG. 19C illustrates a beam profile of the light beam emitted by anexample of the micro-LED device of FIG. 19A, where bonding layer 1940may include SiO₂, the thickness of bonding layer 1940 on top of secondmesa structure 1912 may be about 25 nm, the width of micro-lens 1950 maybe about 2 μm, the width of the active region may be less than 1 μm(e.g., about 0.8 μm), and the width of second mesa structure 1912 may beabout 0.6 μm. A curve 1960 shows the power density of the extractedlight as a function of the emission angle for emitted light of a singlewavelength (e.g., about 625 nm). A curve 1970 shows the spectrallyintegrated power density of the extracted light as a function of theemission angle, where the FWHM of the spectrum of the emitted light maybe about 20 nm. The total light extraction efficiency for extractedlight with emission angles within ±90° may be about 28.7%, and thecollected light extraction efficiency for extracted light with emissionangles within ±18.5° may be about 2.4%.

FIG. 19D illustrates a beam profile of the light beam emitted by anexample of the micro-LED device of FIG. 19A, where bonding layer 1940may include SiN, the thickness of bonding layer 1940 on top of secondmesa structure 1912 may be about 25 nm, the width of micro-lens 1950 maybe about 2 μm, the width of the active region may be less than 1 μm(e.g., about 0.8 μm), and the width of second mesa structure 1912 may beabout 0.6 μm. A curve 1962 shows the power density of the extractedlight as a function of the emission angle for emitted light of a singlewavelength (e.g., about 625 nm). A curve 1972 shows the spectrallyintegrated power density of the extracted light as a function of theemission angle, where the FWHM of the spectrum of the emitted light maybe about 20 nm. The total light extraction efficiency for extractedlight with emission angles within ±90° may be about 32.4%, and thecollected light extraction efficiency for extracted light with emissionangles within ±18.5° may be about 2.2%.

Table 1 below summarizes the simulation results for the micro-LEDdevices shown in, for example, FIGS. 12D, 13A, 14A, 15B, 18A, and 19A.As indicated in Table 1, in the example shown in FIG. 19A, the collectedlight extraction efficiency can be improved by five times over theexample shown in FIG. 12D or 13 , and can be improved by 1.6 times overthe example shown in FIG. 14A.

TABLE 1 Simulation results for micro-LED devices having differentstructures Total LEE in Gain in 18.5° LEE Gain in 18.5° LEE 18.5° overno lens over Nitride lens No lens  4.0% 0.4% N.A. N.A. Nitride lens 5.8% 0.9% 2.24 N.A. USG bonding layer without a step 18.1% 1.4% 3.5 1.5USG bonding layer with 25 nm 28.7% 2.4% 6 2.6 central section with 0.6um diameter step Si₃N₄ bonding layer with 25 nm 32.4% 2.2% 5.5 2.4central section with 0.6 um diameter step

Embodiments disclosed herein may be used to implement components of anartificial reality system or may be implemented in conjunction with anartificial reality system. Artificial reality is a form of reality thathas been adjusted in some manner before presentation to a user, whichmay include, for example, a virtual reality, an augmented reality, amixed reality, a hybrid reality, or some combination and/or derivativesthereof. Artificial reality content may include completely generatedcontent or generated content combined with captured (e.g., real-world)content. The artificial reality content may include video, audio, hapticfeedback, or some combination thereof, and any of which may be presentedin a single channel or in multiple channels (such as stereo video thatproduces a three-dimensional effect to the viewer). Additionally, insome embodiments, artificial reality may also be associated withapplications, products, accessories, services, or some combinationthereof, that are used to, for example, create content in an artificialreality and/or are otherwise used in (e.g., perform activities in) anartificial reality. The artificial reality system that provides theartificial reality content may be implemented on various platforms,including an HMD connected to a host computer system, a standalone HMD,a mobile device or computing system, or any other hardware platformcapable of providing artificial reality content to one or more viewers.

FIG. 20 is a simplified block diagram of an example electronic system2000 of an example near-eye display (e.g., HMD device) for implementingsome of the examples disclosed herein. Electronic system 2000 may beused as the electronic system of an HMD device or other near-eyedisplays described above. In this example, electronic system 2000 mayinclude one or more processor(s) 2010 and a memory 2020. Processor(s)2010 may be configured to execute instructions for performing operationsat a number of components, and can be, for example, a general-purposeprocessor or microprocessor suitable for implementation within aportable electronic device. Processor(s) 2010 may be communicativelycoupled with a plurality of components within electronic system 2000. Torealize this communicative coupling, processor(s) 2010 may communicatewith the other illustrated components across a bus 2040. Bus 2040 may beany subsystem adapted to transfer data within electronic system 2000.Bus 2040 may include a plurality of computer buses and additionalcircuitry to transfer data.

Memory 2020 may be coupled to processor(s) 2010. In some embodiments,memory 2020 may offer both short-term and long-term storage and may bedivided into several units. Memory 2020 may be volatile, such as staticrandom access memory (SRAM) and/or dynamic random access memory (DRAM)and/or non-volatile, such as read-only memory (ROM), flash memory, andthe like. Furthermore, memory 2020 may include removable storagedevices, such as secure digital (SD) cards. Memory 2020 may providestorage of computer-readable instructions, data structures, programmodules, and other data for electronic system 2000.

In some embodiments, memory 2020 may store a plurality of applicationmodules 2022 through 2024, which may include any number of applications.Examples of applications may include gaming applications, conferencingapplications, video playback applications, or other suitableapplications. The applications may include a depth sensing function oreye tracking function. Application modules 2022-2024 may includeparticular instructions to be executed by processor(s) 2010. In someembodiments, certain applications or parts of application modules2022-2024 may be executable by other hardware modules 2080. In certainembodiments, memory 2020 may additionally include secure memory, whichmay include additional security controls to prevent copying or otherunauthorized access to secure information.

In some embodiments, memory 2020 may include an operating system 2025loaded therein. Operating system 2025 may be operable to initiate theexecution of the instructions provided by application modules 2022-2024and/or manage other hardware modules 2080 as well as interfaces with awireless communication subsystem 2030 which may include one or morewireless transceivers. Operating system 2025 may be adapted to performother operations across the components of electronic system 2000including threading, resource management, data storage control and othersimilar functionality.

Wireless communication subsystem 2030 may include, for example, aninfrared communication device, a wireless communication device and/orchipset (such as a Bluetooth® device, an IEEE 802.11 device, a Wi-Fidevice, a WiMax device, cellular communication facilities, etc.), and/orsimilar communication interfaces. Electronic system 2000 may include oneor more antennas 2034 for wireless communication as part of wirelesscommunication subsystem 2030 or as a separate component coupled to anyportion of the system. Depending on desired functionality, wirelesscommunication subsystem 2030 may include separate transceivers tocommunicate with base transceiver stations and other wireless devicesand access points, which may include communicating with different datanetworks and/or network types, such as wireless wide-area networks(WWANs), wireless local area networks (WLANs), or wireless personal areanetworks (WPANs). A WWAN may be, for example, a WiMax (IEEE 802.16)network. A WLAN may be, for example, an IEEE 802.11x network. A WPAN maybe, for example, a Bluetooth network, an IEEE 802.15x, or some othertypes of network. The techniques described herein may also be used forany combination of WWAN, WLAN, and/or WPAN. Wireless communicationssubsystem 2030 may permit data to be exchanged with a network, othercomputer systems, and/or any other devices described herein. Wirelesscommunication subsystem 2030 may include a means for transmitting orreceiving data, such as identifiers of HMD devices, position data, ageographic map, a heat map, photos, or videos, using antenna(s) 2034 andwireless link(s) 2032.

Embodiments of electronic system 2000 may also include one or moresensors 2090. Sensor(s) 2090 may include, for example, an image sensor,an accelerometer, a pressure sensor, a temperature sensor, a proximitysensor, a magnetometer, a gyroscope, an inertial sensor (e.g., a modulethat combines an accelerometer and a gyroscope), an ambient lightsensor, or any other similar module operable to provide sensory outputand/or receive sensory input, such as a depth sensor or a positionsensor.

Electronic system 2000 may include a display module 2060. Display module2060 may be a near-eye display, and may graphically present information,such as images, videos, and various instructions, from electronic system2000 to a user. Such information may be derived from one or moreapplication modules 2022-2024, virtual reality engine 2026, one or moreother hardware modules 2080, a combination thereof, or any othersuitable means for resolving graphical content for the user (e.g., byoperating system 2025). Display module 2060 may use LCD technology, LEDtechnology (including, for example, OLED, ILED, μ-LED, AMOLED, TOLED,etc.), light emitting polymer display (LPD) technology, or some otherdisplay technology.

Electronic system 2000 may include a user input/output module 2070. Userinput/output module 2070 may allow a user to send action requests toelectronic system 2000. An action request may be a request to perform aparticular action. For example, an action request may be to start or endan application or to perform a particular action within the application.User input/output module 2070 may include one or more input devices.Example input devices may include a touchscreen, a touch pad,microphone(s), button(s), dial(s), switch(es), a keyboard, a mouse, agame controller, or any other suitable device for receiving actionrequests and communicating the received action requests to electronicsystem 2000. In some embodiments, user input/output module 2070 mayprovide haptic feedback to the user in accordance with instructionsreceived from electronic system 2000. For example, the haptic feedbackmay be provided when an action request is received or has beenperformed.

Electronic system 2000 may include a camera 2050 that may be used totake photos or videos of a user, for example, for tracking the user'seye position. Camera 2050 may also be used to take photos or videos ofthe environment, for example, for VR, AR, or MR applications. Camera2050 may include, for example, a complementary metal-oxide-semiconductor(CMOS) image sensor with a few millions or tens of millions of pixels.In some implementations, camera 2050 may include two or more camerasthat may be used to capture 3-D images.

In some embodiments, electronic system 2000 may include a plurality ofother hardware modules 2080. Each of other hardware modules 2080 may bea physical module within electronic system 2000. While each of otherhardware modules 2080 may be permanently configured as a structure, someof other hardware modules 2080 may be temporarily configured to performspecific functions or temporarily activated. Examples of other hardwaremodules 2080 may include, for example, an audio output and/or inputmodule (e.g., a microphone or speaker), a near field communication (NFC)module, a rechargeable battery, a battery management system, awired/wireless battery charging system, etc. In some embodiments, one ormore functions of other hardware modules 2080 may be implemented insoftware.

In some embodiments, memory 2020 of electronic system 2000 may alsostore a virtual reality engine 2026. Virtual reality engine 2026 mayexecute applications within electronic system 2000 and receive positioninformation, acceleration information, velocity information, predictedfuture positions, or any combination thereof of the HMD device from thevarious sensors. In some embodiments, the information received byvirtual reality engine 2026 may be used for producing a signal (e.g.,display instructions) to display module 2060. For example, if thereceived information indicates that the user has looked to the left,virtual reality engine 2026 may generate content for the HMD device thatmirrors the user's movement in a virtual environment. Additionally,virtual reality engine 2026 may perform an action within an applicationin response to an action request received from user input/output module2070 and provide feedback to the user. The provided feedback may bevisual, audible, or haptic feedback. In some implementations,processor(s) 2010 may include one or more GPUs that may execute virtualreality engine 2026.

The methods, systems, and devices discussed above are examples. Variousembodiments may omit, substitute, or add various procedures orcomponents as appropriate. For instance, in alternative configurations,the methods described may be performed in an order different from thatdescribed, and/or various stages may be added, omitted, and/or combined.Also, features described with respect to certain embodiments may becombined in various other embodiments. Different aspects and elements ofthe embodiments may be combined in a similar manner. Also, technologyevolves and, thus, many of the elements are examples that do not limitthe scope of the disclosure to those specific examples.

Specific details are given in the description to provide a thoroughunderstanding of the embodiments. However, embodiments may be practicedwithout these specific details. For example, well-known circuits,processes, systems, structures, and techniques have been shown withoutunnecessary detail in order to avoid obscuring the embodiments. Thisdescription provides example embodiments only, and is not intended tolimit the scope, applicability, or configuration of the invention.Rather, the preceding description of the embodiments will provide thoseskilled in the art with an enabling description for implementing variousembodiments. Various changes may be made in the function and arrangementof elements without departing from the spirit and scope of the presentdisclosure.

Also, some embodiments were described as processes depicted as flowdiagrams or block diagrams. Although each may describe the operations asa sequential process, many of the operations may be performed inparallel or concurrently. In addition, the order of the operations maybe rearranged. A process may have additional steps not included in thefigure. Furthermore, embodiments of the methods may be implemented byhardware, software, firmware, middleware, microcode, hardwaredescription languages, or any combination thereof.

Terms, “and” and “or” as used herein, may include a variety of meaningsthat are also expected to depend at least in part upon the context inwhich such terms are used. Typically, “or” if used to associate a list,such as A, B, or C, is intended to mean A, B, and C, here used in theinclusive sense, as well as A, B, or C, here used in the exclusivesense. In addition, the term “one or more” as used herein may be used todescribe any feature, structure, or characteristic in the singular ormay be used to describe some combination of features, structures, orcharacteristics. However, it should be noted that this is merely anillustrative example and claimed subject matter is not limited to thisexample. Furthermore, the term “at least one of” if used to associate alist, such as A, B, or C, can be interpreted to mean A, B, C, or anycombination of A, B, and/or C, such as AB, AC, BC, AA, ABC, AAB,AABBCCC, etc.

The specification and drawings are, accordingly, to be regarded in anillustrative rather than a restrictive sense. It will, however, beevident that additions, subtractions, deletions, and other modificationsand changes may be made thereunto without departing from the broaderspirit and scope as set forth in the claims. Thus, although specificembodiments have been described, these are not intended to be limiting.Various modifications and equivalents are within the scope of thefollowing claims.

What is claimed is:
 1. A light source comprising: an array ofmicro-light emitting diodes (micro-LEDs), each micro-LED of the array ofmicro-LEDs including a first mesa structure formed in a plurality ofsemiconductor layers; an array of micro-lenses; and a bonding layerbonding the array of micro-lenses to a first semiconductor layer of theplurality of semiconductor layers, wherein the first semiconductor layerincludes an array of second mesa structures formed therein, each secondmesa structure of the array of second mesa structures being between arespective micro-lens of the array of micro-lenses and the first mesastructure of a respective micro-LED of the array of micro-LEDs.
 2. Thelight source of claim 1, wherein a refractive index of the array ofmicro-lenses is equal to or greater than a refractive index of the firstsemiconductor layer.
 3. The light source of claim 1, wherein an opticalthickness of the bonding layer between each second mesa structure and acorresponding micro-lens of the array of micro-lenses is less than ⅕ ofa center wavelength of light emitted by the array of micro-LEDs.
 4. Thelight source of claim 1, wherein a maximum optical thickness of thebonding layer is equal to an integer multiple of a half-wavelength of acenter wavelength of light emitted by the array of micro-LEDs.
 5. Thelight source of claim 1, wherein: a width of the second mesa structureis smaller than a width of the first mesa structure; and the second mesastructure includes vertical or tilted sidewalls.
 6. The light source ofclaim 1, wherein a refractive index of the bonding layer is lower than arefractive index of the first semiconductor layer.
 7. The light sourceof claim 1, wherein the bonding layer includes SiO₂, SiN, or atransparent conductive oxide (TCO).
 8. The light source of claim 1,wherein the array of micro-lenses and the first semiconductor layerinclude AlGaInP.
 9. The light source of claim 1, wherein: a pitch of thearray of micro-LEDs is equal to or less than 2 μm; a width of the firstmesa structure is equal to or less than 1.2 μm; and a width of thesecond mesa structure is equal to or less than 0.8 μm.
 10. The lightsource of claim 1, wherein a ratio between a width of a micro-lens ofthe array of micro-lenses and a width of the second mesa structure isgreater than
 2. 11. The light source of claim 1, wherein the pluralityof semiconductor layers includes: a p-doped semiconductor layer; anactive layer configured to emit visible light; and an n-dopedsemiconductor layer, wherein the first semiconductor layer includes thep-doped semiconductor layer or the n-doped semiconductor layer.
 12. Thelight source of claim 1, wherein each second mesa structure of the arrayof second mesa structures is at a focal point of the respectivemicro-lens of the array of micro-lenses.
 13. The light source of claim1, wherein each micro-LED of the array of micro-LEDs includes: apassivation layer on sidewalls of the first mesa structure; and a backreflector coupled to a second semiconductor layer of the plurality ofsemiconductor layers and electrically connected to a backplane wafer.14. A micro-light emitting diode (micro-LED) device comprising: abackplane wafer including electrical circuits fabricated thereon; anarray of micro-LEDs bonded to the backplane wafer, each micro-LED of thearray of micro-LEDs including: a first mesa structure formed in a firstside of a plurality of semiconductor layers facing the backplane wafer;and a second mesa structure in a second side of the plurality ofsemiconductor layers, wherein a center of the second mesa structure isaligned with a center of the first mesa structure; an array ofmicro-lenses; and a bonding layer bonding the array of micro-lenses tothe second mesa structures of the array of micro-LEDs, wherein thesecond mesa structures of the array of micro-LEDs are between the firstmesa structures of the array of micro-LEDs and the array ofmicro-lenses.
 15. The micro-LED device of claim 14, wherein a refractiveindex of the array of micro-lenses is equal to or greater than arefractive index of the plurality of semiconductor layers.
 16. Themicro-LED device of claim 14, wherein an optical thickness of thebonding layer between the second mesa structure and a correspondingmicro-lens of the array of micro-lenses is less than ⅕ of a centerwavelength of light emitted by the array of micro-LEDs.
 17. Themicro-LED device of claim 14, wherein a maximum optical thickness of thebonding layer is equal to an integer multiple of a half-wavelength of acenter wavelength of light emitted by the array of micro-LEDs.
 18. Themicro-LED device of claim 14, wherein a refractive index of the bondinglayer is lower than a refractive index of the plurality of semiconductorlayers.
 19. The micro-LED device of claim 14, wherein: a width of thesecond mesa structure is smaller than a width of the first mesastructure; and a ratio between a width of a micro-lens of the array ofmicro-lenses and the width of the second mesa structure is greater than2.
 20. The micro-LED device of claim 14, wherein the second mesastructure is at a focal point of a corresponding micro-lens of the arrayof micro-lenses.